A distinctive crustose species with black thallus that consists of very small, granular-looking squamules that are often gathered in areoles. The thallus is limited by a dark blue, wide, felted prothallus. The thallus maybe be covered by a grey pruina. If present, the black apothecia have a glossy disc.
In Scotland, Placynthium nigrum has a scattered distribution and is found mainly on rather hard, but damp, calcareous substrata or mortar and concrete. BLS distribution map
scottishlichens.co.uk 