Blue cheese is a general classification of cow's milk, sheep's milk, or goat's milk cheeses that has ha Penicillium cultures added so that the final product is spotted or veined throughout with blue or blue-green mold, and carries a distinct smell. Some blue cheeses are injected with spores before the curds form and others have spores mixed in with the curds after they form. Blue cheeses are typically aged in a temperature-controlled environment such as a cave.