I would tell him the blunt truth. Avoiding the whole truth is like lying. It shows that we fear them. It gives them power they do not deserve. You do not need his permission to get help for yourself and your kids. You do not owe him or anyone else an explanation.
That only holds if AspieGirl's husband has accepted a diagnosis of BPD.
If he does not admit having BPD, the consensus is that it is futile to express to him that he has it.
If he admits it, then certainly she can specify what kind of support group it is.
If he does NOT admit it, she need not be specific as it would be needlessly provocative to him.