Although Google gives 914 references to the statement
"I am not blonde"
and but 214 to "I am not blond"
both spelllings if tge word are correct.
Check dictionary.com - 4 listings for "blonde", but 5 for "blond"
I always thought it was because of a probable French origin, with the e added to make it feminine, but I didn't look it up, so I am not sure.
Also did not check to see if the "blonde" variant (IAmNotBlonde.com) was available; you should, of course, have both if you use it at all.
Oh, also IAmNotBlonndd.com for actual blondes/blonds, I guess. (Just kidding : )
Added:
Just saw post above mine, sent while I was writing - glad to see a lot of us know this and bother to respond about it.