I think it's against the law to provide a negative reference.
I think you're right, at least in NY. It's like the rule your mother always told you, "If you don't have anything nice to say, don't say anything at all."

If your ex-boss did berate you to this new place and your laws are the same as NY, you could have a libel case against her...Before you tell these new people to shove it, I would try to find out what was said to them, if anything by the nasty ex-boss, get it in writing if you can.