Inkstain bolete: the bolete that turns black when cut — photos

The inkstain bolete (*Cyanoboletus pulverulentus*) is a pored mushroom with a brown cap, bright yellow pores and a yellow stem that turns blue-black within seconds at the slightest touch and when cut, as if someone had spilled ink on it. It grows in broadleaf and mixed woods under oak, beech, hornbeam and other trees, from summer into autumn. Blueing on its own does not mean a mushroom is poisonous, but researchers have found a great deal of arsenic in this one, so eating it is not recommended.

We came across it at the end of May 2026 in a mixed wood where pine grows together with red oak and hornbeam. A large mushroom stood in wet leaves, with a young one peeping out beside it. We picked it up, and within a minute the stem was covered in dark fingerprints. And when we cut it, the cut surface went black before our eyes.

What the inkstain bolete looks like

Here are the main features of the inkstain bolete:

• Cap 4–10 cm, convex in young mushrooms, later cushion-shaped. The colour varies: from yellowish ochre and light brown to dark brown and olive, sometimes with a reddish tinge. In young mushrooms the surface is dry and faintly velvety, as if dusted with powder.
• Pores lemon yellow, instantly bruising blue-black when touched.
• Stem 4–8 cm tall, cylindrical, bright yellow at the top, reddish brown towards the base, usually without a net pattern.
• Flesh yellow, turning blue-black within seconds when cut. Smell and taste indistinct.
• Spore print olive brown.

In the first photo our mushroom has just been found; we only tilted it to show the pores. The cap is ochre brown, the pores bright yellow, and on the yellow stem blue-black patches are already darkening where fingers touched it. The lower part of the stem is covered in soil and the same dark patches. On the left stands a young inkstain bolete with a rounded little cap.

The cut turns black in seconds

This mushroom gives itself away best under the knife. In many xerocomus boletes the cut flesh blues weakly or slowly. In the inkstain bolete the yellow flesh floods with blue within seconds, and the blue is so dense that it looks almost black. Its English name says exactly that: “inkstain bolete”.

In the photo is the same large mushroom that we cut. We turned the cap over: the pores are still lemon yellow where nobody touched them, but the cut stem is already solid ink. The young mushroom beside it has also darkened from being handled.

It is not only the flesh that darkens. The cap reacts to touch as well, though less vividly than the pores and stem. Look at what the cap was like when we had just picked the mushroom: ochre, with a few dark fingerprints. A small young mushroom is also visible on the leaves beside it.

And these are the same mushrooms from above twenty minutes later, now by a stump in the moss. The big one's cap has darkened and become covered in brownish-black blotches, and the young one has turned almost dark grey. So if you come across a “dark” inkstain bolete, perhaps someone has simply held it in their hands already.

Why it turns blue, and is that a sign of poison

The flesh of many boletes contains yellowish pigments from the pulvinic acid group, chiefly variegatic and xerocomic acids. When the mushroom is cut or bruised, these acids meet oxygen in the air and oxidise into blue compounds. It is a chemical reaction to damage, not a signal saying “I'm poisonous”.

Very good edible mushrooms turn blue too, for example the bay bolete, and so do mushrooms you must not eat. So the colour of the cut tells you nothing about safety. We wrote about this in more detail in the article why xerocomus boletes turn blue when cut, and about the boletes that blue most strongly in the piece blue-bruising boletes.

Even the genus's scientific name is about this. *Cyanoboletus* means “blue bolete”, from the Greek “kyanos”, blue. The species name *pulverulentus* translates as “powdered” and refers to the dry, dusted-looking surface of the young cap and stem. The mushroom was described back in 1836 as *Boletus pulverulentus*, later placed among the xerocomus boletes of the genus *Xerocomus*, which is where its Ukrainian name comes from, and in 2014 it became the type species of the new genus *Cyanoboletus*.

Where and when the inkstain bolete grows

The inkstain bolete forms mycorrhiza with trees, meaning it lives in partnership with their roots. It is most often found under beech and oak, but DNA analysis shows that its partners can also include hornbeam, hazel, lime, sweet chestnut and even Scots pine. It grows singly or in small groups in broadleaf and mixed woods, in parks and along old avenues.

Field guides give its season as June to November, and in Britain July to October. Our end-of-May find was an early one, but European researchers also mention finds in May. It occurs in Ukraine too, though it is not often seen.

To see the mushroom from the side, we laid it on a green leaf right on the forest floor. The cap is ochre, the pores lemon yellow, and on the yellow stem there is a blue-black patch where fingers held it.

Its corner of the forest is in the photo at the top of the article: a mossy stump, low moss and a large leaf beside it. We had already cut the stem of the big mushroom; on the left stands the young one, and both have darkened from our touch.

Is the inkstain bolete edible?

Let's be honest from the start: many field guides still list it as edible, with middling flavour. But in 2018 Czech and Austrian researchers tested 39 collections of this mushroom, mostly from the Czech Republic, and found that it accumulates arsenic. The median was 160 mg per kilogram of dry weight, and the maximum reached 1300 mg. For comparison, mycorrhizal mushrooms from clean sites rarely contain more than 10 mg.

What matters about this finding:

• The arsenic was in the mushrooms regardless of the soil. The researchers found no link between the arsenic content of the soil and that of the mushroom, so a “clean forest” guarantees nothing here.
• Most of it is in the tubes. In two mushrooms analysed in detail, about 80 % of the arsenic sat in the tube layer under the cap, 11–16 % in the cap flesh and 3–9 % in the stem.
• The arsenic is in organic form. It is dimethylarsinic acid. Its acute toxicity is much lower than that of inorganic arsenic, but the US Environmental Protection Agency considers it a probable carcinogen.
• This is a trait of this particular species. In 2026, researchers who revised the genus *Cyanoboletus* confirmed that among boletes only the inkstain bolete is known to accumulate arsenic, while its close relatives that were tested contain many times less.

The conclusion of the 2018 study's authors: the inkstain bolete should not be recommended as an edible mushroom, and if you do eat it, then no more than 90 g of fresh mushrooms a year. The Swiss Association of Official Mushroom Inspectors (VAPKO) is more lenient: it still allows it as an edible mushroom, but with restrictions: no more than 100 g per meal, always cut away the tube layer, and eat this mushroom only a few times a year. We wouldn't put it in the basket for the frying pan: it isn't known for its flavour, and gathering enough for a dish is hard anyway, because it usually grows singly.

What the inkstain bolete can be confused with

Several boletes blue strongly, and at first glance they're easy to mix up. Look at the pores and the stem:

• Scarletina bolete also instantly turns dark blue when touched, and its cap is brown too. But its pores are orange or red, and the stem is covered in tiny red dots. It is conditionally edible and needs thorough cooking.
• Lurid bolete has orange-red pores, a red net on the stem and a wine-red line between the tubes and the cap flesh. It grows under beech and oak and is conditionally edible.
• Suede bolete has an olive-brown velvety cap and yellow pores, but it blues only weakly, and the cut flesh of the stem barely changes colour.
• Bay bolete with its chestnut cap: its pores are pale yellow and blue when pressed, but the cut flesh blues only slightly. It prefers conifer woods, though it also turns up under broadleaf trees.

Yellow pores, a yellow stem without red dots or a net, plus an inky reaction within seconds — that is the portrait of the inkstain bolete. If the pores are bright orange or red, the mushroom belongs to a different crowd, one that includes poisonous species too. We covered them in the article the poisonous mushroom that looks like porcini, and other xerocomus lookalikes in the piece xerocomus lookalikes.

The inkstain bolete is one of the most striking mushrooms for getting to know the forest. Hold it in your hands, watch the yellow turn to ink, take a photo. But for the frying pan, look for other xerocomus boletes instead.