Well, I AM dating a black belt who also grapples (UFC)...there are lots of reasons I am with him but these are black-belt karate training/grappling related.
1) Discipline: he has had to train to get this far and for competitions which takes a lot of mental and physical discipline and dedication. I know that he can finish any task which he sets himself up to do.
2) Balance: Karate teaches balance (mental balance and phsycial balance). Being able to see both sides of a situation is very helpful and makes discussions about things accomplish more.
3) MMMMM: Most people I know who practice karate work out regularly so his body is mm mm good.
4) He has a hobby: training usually takes up one or two nights a week in addition to his regular work outs...and he has a lot of friends from karate and grappling so he isn't just bummin' around the house after work all the time. It's nice to both have stuff to do (then you won't feel guilty going out w/ the girls!)
5) He can kick anyone's ***: Most people w/ a lot of karate training won't get all wild b/c they respect physical strength and don't abuse fighting...but if someone pissed him off or tried to come at me the guy wouldn't be able to walk after. That's kind of hot.
6) He isn't cocky: He knows he can fight and usually wins...but also knows his limitations so he doesn't walk around trying to prove himself - b/c he doens't have to.
Well that's all I can do for black belt stuff. If I think of anything else I'll definately add it!