Yes, I do believe I have a mental block! Either that, or there is just some part of my anatomy in the whole chain of events that must happen to do a chin-up that is the "weak link." Or I just haven't yet found the "groove" I have to get into to do them. (I think my inner "kid who couldn't even do one bent-arm hang" is showing herself).
Right now, I'm working on healing a chronic shoulder problem I've had off and on for years, so I can't try it yet (no heavy shoulder work for a while, and I'm afraid chin-ups would be too much for now)but I'll have to try negatives (I knew about them, but just never got into them).