What are the small black or blue-green moulds on my substrate?

Small black tufts are usually Aspergillus or a black pin mould, and blue-green patches are often Penicillium, the same family as the mould on old bread. Both arrive as airborne spores landing on under-colonised or rich substrate. Some can release irritating spores, so seal and remove the material without disturbing it, and improve sterilising and air cleanliness.