During bacteriological analysis of water and sediments from pond shrimp I have seen black round colonies grown on TCBS and TSA supplemented with 2.0% NaCl
Bacteria that are not Vibrio but produce hydrogen sulfide grow as small black colonies. This is because the hydrogen sulphide produced from thiosulphate, which acts as a source of sulphur and creates a reduced oxygen tension in which Vibrio can grow due to its facultative anaerobic nature, combines with ferric ions from ferric citrate to produce ferric sulphide, which is black (https://en.wikipedia.org/wiki/Thiosulfate-citrate-bile_salts-sucrose_agar)