Blackspot, or melanosis, in shrimp is a harmless but objectionable discoloration or darkening, occurring primarily along the swimmerets, head, tail and nearby shell areas, before spreading further along shell edges and through the body.
Blackspot is caused by a system of enzymes that are naturally present in the shrimp. These enzymes, in the presence of air, can chemically transform colorless compounds in the shrimp into complex brown pigments near the shrimp surfaces and shell.