Catillaria chalybaea (Borrer) A. Massal.
A widespread species of siliceous rocks. The shiny, black, sessile apothecia on a thin, grey, smooth thallus are distinctive. It is a variable species and the greenish-grey thallus can be quite ephemeral
Smith et al. editors (2009) The Lichens of Great Britain and Ireland
Dobson (2018) Lichens: an Illustrated Guide to the British and Irish Species