So I was hitting it off with this chick at a bar and she asked the question, "which bartender is hotter". Now there were two, one was a petite brunette with long hair, the other a thin busty blonde with shorter hair both were HB9. I cant recall exactly what I said, but the next day I thought about a good response if this were to ever happen again.
This is what I should have said.- "Well the brunette is bangin but she does remind me of my ex just smaller boobs (dhv) and I dont want to go down that path haha, but the other one is blonde and I'm not really a fan of blondes, so I dont know if I'd take either" Now the chick who asked me this question was blonde so I feel that is a good placement of a neg, just twist to your situation. Naturally the chick will ask you what you have against blondes or whatever her hair color is you just negged, then improvise from there.
I myself am quite bold when talking to women and would go with something like "blondes just never do it for me in bed, Im sorry but for all the hype about you guys your pretty boring" being dead serious (not cocky funny). See where that goes...Just something to have in your guys' back pocket. Good Luck.