Description: Thallus foliose, adnate, forming 2-5 cm wide, orbicular or irregular rosettes. Lobes up to c. 2.5 cm long, 0.5-1(-2) mm broad, more or less convex, stiff, contiguous or overlapping; center of thallus with narrower (0.2-0.4 mm), flattened secondary lobules which arise perpendicularly to the surface. Upper surface pale grey to brownish, darkening or blackening especially on the lobules and towards the tips, and there becoming shiny, reticulate-maculate at least in the apical 2-3 mm. Lower surface black, brown toward the tips, smooth to weakly and shallowly wrinkled, with scattered hapters. Upper cortex paraplectenchymatous, the cell walls with Cetraria-type lichenan, with a non-pored epicortex; medulla white, solid; lower cortex paraplectenchymatous. Apothecia rare, lecanorine, up to 5(-7) mm across, with a dark brown disc and a thin, entire to sublobulate thalline margin. Epithecium brownish; hypothecium and hymenium colourless. Asci 8-spored, clavate, the K/I+ blue tholus penetrated by a faintly amyloid apical cushion with parallel or diverging flanks, the wall K/I-, surrounded by a K/I+ blue outer layer, Lecanora-type. Ascospores 1-celled, hyaline, subglobose to broadly ellipsoid, 10-12 x 6-8 µm. Pycnidia numerous, immersed, the visible part black to grey-brown. Conidia bifusiform, 5-7 x c. 1 µm. Photobiont chlorococcoid. Spot tests: upper cortex K+ yellow, C-, KC-, P-; medulla K+ slowly reddish to orange-brown, C-, KC-, P+ orange, UV-. Chemistry: upper cortex with atranorin and chloroatranorin; medulla with fumarprotocetraric acid.
Note: an arctic-alpine to boreal-montane, circumpolar lichen found in more sheltered and less wind-exposed situations than B. atrofusca, on faces of acid siliceous rocks with a long snow-cover, with optimum above treeline; most frequent in the Alps, but also present in the high Mediterranean mountains.