Reasons for blackening:
Shrimp contains enzymes and tyrosine, which can still maintain its activity in the cold chain process. If the temperature changes and it is exposed to oxygen, the oxidase (phenolase or phenoloxidase) in shrimp will oxidize tyrosine and produce melanin, which will lead to the blackening of the head and tail of shrimp, and even the whole shrimp will be browned, and even deteriorate seriously.