Shrimps with heavy black on their back stripes...(now updated with pics)

Tiger Prawns

Talking about one of these?

Could be iqf shrimp that have been defrosted and starting to go bad, what exactly do you mean by heavy black on the back stripes? Pic?

If you are getting several every time you buy, then it's probably not black tigers. They aren't that abundant yet.

It's probably milk shrimp or cotton disease. It's a parasite that grows in shrimp that makes the shrimp turn black and white. I usually throw them away if it affects(makes it turn white) the meat.

Cotton Disease

Like this?

That would be Melanosis. It's the natural/harmless breakdown of enzymes. Shrimpers commonly use sulfating agents to prevent it. Perfectly safe to eat.