The blackening waxcap, Hygrocybe conica, goes jet black throughout when old…but the shape and general appearance of this fungus suggest a blue-black Entoloma species to me, possibly Entoloma chalybeum which has bluish gills. Both occur in unimproved grassland, but H. conica would be the more suitable species for Hollowe’en since the Americans actually call it “witch’s hat”…
