The basic responses I found on-line were:
- Return with another backhanded compliment (this one didn't really fill the bill, as it's not particularly gracious)
- A polite call-out of the backhanded nature of the "compliment", for example "That's not really a good thing."
- An acceptance with a polite call-out, for example "I'll accept your left-handed compliment with my right hand, thank you."
- An acceptance with positive attention to the target of the implied insult, for example "Thank you! I think (x) was pretty good too / wasn't bad either!"
Update with related post, roughly 2 years later: Christian response to backhanded compliments: Reprise.