It's always interesting when a new black belt arrives from outside the school. On Monday, I met this new sempai, and he was a credit to his belt--he was polite, respectful and amazingly skilled. I would guess he's in his late 20s; he's training with my sensei for competitive fights, and he's going to be in our school for a while.
He could tie me in knots in grappling, but he was deliberate and safe in his movements--he stopped at one point when I grunted to make sure I was okay. (I make a lot of noise when I grapple, unfortunately!)
I'm sure I'll learn a lot grappling with him, I just have to retain it in my head. One of the most challenging things about grappling is retaining/calling up when necessary what I've learned--it's a very technically complex art. There's also the challenge of embedding what I've learned into muscle memory.
Standup (kickboxing) also has a muscle-memory element. But much more so than in grappling, the biggest challenge for me is getting past sensory and emotional overload (excitement, fear) in order to do what I have learned to do.