Well, there is actually more evidence to support Achilles being blonde rather than dark haired. In the Iliad, Achilles' hair is described as golden/blonde or fair. Furthermore, many gods and goddesses, especially Demeter and Aphrodite, were depicted as having blonde/golden hair and blue/bright eyes; they were two characteristics that usually accompanied each other. Achilles is described as being bright-eyed in The Iliad, so it's safe to assume he probably had blonde/golden hair. Now, because Aphrodite was blonde and the goddess of love of beauty, blonde/golden hair was considered beautiful and a blessing from the gods, Greeks used saffron and mud to dye their hair a lighter colour. Because Achilles was both beautiful and divine, golden/blonde hair is probably the fairest representation of him.