I've rolled with 7 different black belts so far. My main BB instructor rolls with me almost everyday for 30 minutes sometimes. So I've thankfully gotten a lot of time with really quality opponents. It's not about how good or how bad you do, but how much you learn. I feel like I get better WHILE rolling with black belts. For me, I did notice I do better or maybe just prefer rolling with purple belts and up because they actually go for real moves. A lot of whites and blues that I know, tend to force through moves they don't fully understand and it can be unpredictable, so I have to react in the moment, where as purple belts and higher, I seem to be able to play 3-4 moves ahead of what's going on atm.