Im just curious because i often see replays where
blackbelts dismember uke with one strike while i have
done moves that should even pack more punch but
uke's joints takes the strike without breaking.
Does the belt color affect the character's "strenght"?

No. Belt is just there to show how experienced a player is. It isn't related to any 'strength'. Everyone has same tori. You may want to first read replay making encyclopedia. Practice and you will eventually learn what joints do and how to get best used out of them.

No I don't think it does.

Players with higher belt ranks is usually better players, therefor able to deal out harder punches thanks to better technique.

Okay, thanks.