Picture taken on the 16th March 2008, near Plana Novella, Garraf Mountains, Olivella, Barcelona, on hard limestone, vertical position, well-lit place.
It is a very common lichen in Catalonia, always on hard limestone.
Crustose lichen, thallus almost white with a very thin black prothallus (see right corner). Apothecia immersed in the rock, regulary arranged but not in clusters, from red-brown to alost black mainly when dry, small, up to 0,5 mm diam., pruinose disc, grey pruina. It looks like a Verrucaria, but paying attention it is easy to see the lecideine apothecia.
See good drawings in Pascale Tiévant (2001) Guide des Lichens, Delachaux et Niestlé, pag. 118.
Serà una espècie molt comuna a Catalunya, però no per això menys fascinant després de veure la teva foto i llegir les teves explicacions. Felicitats per la captura.