Well Donna, two days later I came up with a good response. Next time (since it's too late for this time and knowing all the idiots out there, there will be a next time) just say in a nice calm voice "I'm sorry you feel that way. I guess that means the black is for all the poor souls who've been unfortunate enough to have crossed your path." Then calmly ask her to please move on as there are other customer's waiting to be checked out.