The "black mark" is a clot of old fixative

Residues of consolidating substances
During previous attempts at restoration, the extent and the seriousness of problems relating to adhesion and cohesion (q.v.) of the painted surfaces in the Chapel, led to the frequent use of substances with adhesive or consolidating properties, both of natural and synthetic origin. The damage caused by such substances varies widely according to their nature and their characteristics during the ageing process. These alterations are not only of a chemical-physical type (such as shrinking, stiffening, reduction of the natural porosity of the painted film), but also of an aesthetic nature, such as yellowing, opaqueness or clouded transparency. In particular, some of the synthetic resins, often applied in thick coats or at too higher degree of concentration, leave frequent traces on the painted surfaces in the form of a semi-transparent film.