[Ahh, Explaining Things. The one area of her life where she's most comfortable.]
Blood color is a very good way that other species delineate "types"! More sensitively to certain cultures, such as the Alternian and Beforan trolls, they're recognized as castes.
Of course, it's not as though that's the only way aliens can exist. Does your reaction mean demons only have cherry red blood, as humans do?