late 15c., from Old French blont "fair, blond" (12c.), from Medieval Latin blundus "yellow," perhaps from Frankish *blund. If it is a Germanic word, it is possibly related to Old English blonden-feax "gray-haired," from blondan, blandan "to mix" (see blend (v.)). According to Littré, the original sense of the French word was "a colour midway between golden and light chestnut," which might account for the notion of "mixed."
Old English beblonden meant "dyed," so it is also possible that the root meaning of blonde, if it is Germanic, may be "dyed," as ancient Teutonic warriors were noted for dying their hair. Du Cange, however, writes that blundus was a vulgar pronunciation of Latin flavus "yellow." Another guess (discounted by German etymologists), is that it represents a Vulgar Latin *albundus, from alba "white."
The word was reintroduced into English 17c. from French, and was until recently still felt as French, hence blonde (with French feminine ending) for females. Italian biondo, Spanish blondo, Old Provençal blon all are of Germanic origin.