Blackening brittlegill or Winecork brittlegill

The cap is 6-20 cm across, hemispherical to flat with an inrolled margin when young, later becoming funnel-shaped. It's white when young, turning dirty grey, ash-grey, then blackish to anthracite black with age. The cap darkens when pressed, unevenly in young specimens. The surface is smooth and slightly sticky in wet weather.

The cap is 5–15 cm across, occasionally up to 20. It starts convex, then flattens, develops a dip in the centre and in old specimens becomes almost funnel-shaped.

Young caps are dirty white or greyish buff, then grey-brown and smoky brown, and dark brown to blackish in old mushrooms. The surface is smooth, slightly sticky and shiny when damp, and the margin is even, without grooves.

The gills are attached to the stem, distant and thick, typical of the genus Russula. They're whitish when young, redden to brownish-black when damaged, and darken with age to grey, then black. The spore print is white.

The gills are narrow and of unequal length, with many short ones in between, sometimes forked, and attached to the stem or running slightly down it. They start white, turn greyish, and darken where pressed. Sources describe the gills as both close and widely spaced, so don't rely on spacing.

The spore print is white.

The stem is 4-8 cm tall and 2-4 cm thick: thick, fleshy, cylindrical and relatively short. It's white when young, turning grey, then blackish with age. The surface is smooth, with no ring.

The stem is 3–6 cm tall, up to 11 cm in large specimens, and 1–3 cm thick. It is hard, cylindrical and smooth, the colour of the cap but paler, and it darkens when handled. The base is often massive, wrinkled and folded.

There is no ring, no volva and no veil remnants.

The flesh is firm, fleshy and white when young. When cut, it goes through a two-phase reaction: first it reddens to greyish or brownish red, then it gradually blackens to anthracite black. This is the classic diagnostic feature of the species. The smell is faintly mushroomy.

The flesh is white, firm and hard. When broken it slowly turns faintly pink over half an hour, then pale smoky grey, but never black as in the blackening brittlegill. There is no milk. The taste is mild and sweetish, sometimes slightly sharp.

Faint, pleasant, mushroomy, with no distinctive notes.

The smell is noticeable and distinctive: musty, like an old empty wine cask, and some describe it as mouldy. This is what gave the mushroom its English name.