It’s not their blood that makes lobsters turn red when they’re cooked.
Lobster blood is colorless unless exposed to oxygen, at which point it develops a bluish tint.
The shells of lobsters are mostly gray, green, or brown when they’re alive, but never red.
Boiling them breaks down the various pigments that color their shells.
Their most durable tints are the reds, so they’re the last to disappear during boiling. Also, they look just so yummy in red.