Spotted this this afternoon before mowing the front lawn, figured I'd get a closer look before the mower eats it and it's friend. There were just the two of them about 5" apart. In the picture of split stipe laid on paper there is an attempt at a spore print in between the two pieces of the stipe. A close look IRL appears to be a white spore print. I've repeated the spore print on foil to see a bit better and it's definitely white (no dark coloured paper handy just now, photo of print on foil not working out well enough to post). In that same shot the black staining is very obvious, it is a couple of hours after cutting it in half. The flesh of the cap has done the same blackening but the gills have not (although I suppose I haven't really damaged the gills anyway).
My best guess is Conical Wax Cap Hygrocybe conica
Edited by Cuboid, 22 June 2019 - 01:04 PM.