It is tough to know what to do in that situation.
I've been trying to picture what I, as the interviewer, would find weirder. If I asked someone how their former boss would describe them, and they went into any amount of detail about how their former boss didn't like them and would say they were a bad person, I would be rather taken aback. It's kind of like going on a blind date and having the person badmouth their ex (that I asked about)--in the absence of other knowledge about the person you're talking to, you don't know if it's them
being paranoid/claiming they've been wronged, or if they're being completely accurate. Of course a lot depends on exactly what is said, tone, body language, etc..
But if someone took my interview question and made it more about their own strengths and weaknesses, perhaps tweaking it slightly in a way that I probably wouldn't even notice ("I hope
she would say..."), that would seem perfectly normal to me. Then, if I actually called that boss, and they started ranting and raving about the person, in a way that did not match my own impression or the other references I'd talked to, it would seem more likely to me that it was the boss who was wacko. Even if they calmly and matter-of-factly said the person was bad, and this didn't match other things I'd heard, it would still just be a question mark, not necessarily a negative. If I invited the person for a second interview, I might mention it to them (if that was allowed by confidentiality) and see what their side of the story was. I personally would not see it as lying/being deceptive in most cases; in fact I might be impressed with their tact.
That's just my take on it. But I've been thinking about it a lot; in the moment I can definitely see how all that wouldn't go through your head in time.