The physical appearance of B. fuscothallinum is marked by a crustose thallus with a continuous, diffuse, effuse pattern and an upper surface of a brownish colour, ranging from pale fawn to mid-cinnamon.

The thallus' upper surface lacks a pruina, a whitish, powdery deposit that covers parts of some lichens, and is generally smooth and devoid of any specific structures.

When present, the apothecia, or spore-producing structures, are small (between 0.2 and 0.3 mm), semi-immersed to emergent, and feature a flat to slightly convex disc of a brownish hue, similar to the thallus' colour.

However, it maintains a symbiotic relationship with a primary photobiont, specifically, a trebouxiaceous, chlorococcoid alga, which supplies the lichen with essential nutrients.

[1] The holotype specimen of B. fuscothallinum was collected by Lücking in French Guiana in 1995, where it was found growing on leaves in a forest understory.