We peeled and cleaned dark blue-gray shrimp. When we steamed them, they turned bright orange. I ask to speak with an authority on shrimp and lobsters. The Internet to the rescue.
Crustaceans look blackish while alive because the orange pink color exists but is hidden.
The chemists get down to the basics of molecular structure to elucidate. The blackish color is due to protein chains. The orange-pink color is a pigment, similar to the one in carrots. The protein chains hide the orange-pink molecules by wrapping around and containing them. Therefore the shell looks blackish.