I agree with Kelleh, black stools/bleeding aren't part of IBS but could be a flare. I'd say definitely get it checked out.
For what it's worth, my GI has also tried to blame my issues on "IBS" when I was supposedly in remission. I don't feel like that's the case, though. When IBD goes into remission, that doesn't mean it's completely gone. It's like the monster is sleeping, but the monster hasn't left. There can be things like scar tissue that are still causing issues even if the inflammation isn't active. Or, it could be an active flare that they just haven't confirmed yet. It seems lazy to me to blame it on IBS, though, especially when you've already got a confirmed IBD.