In our case specifically, it's the colour of his skin. Our black friend and me have almost the same dick size (17 centimeters long and quite fat). Of course he has more experience fucking than me. Moreover he's 8 years older than me. But my wife ever said she liked black men. I'm a caucasian blonde hair man and the reason she married with me was due I'm a good guy.
Everytime they had sex, Axel did everything necessary to get a good rub with his penis over Susie's pussy. One of the last time he fucked Susie, they did it over our bed in doggy. Some hours before Susie and me had been fucking in the same position and I had not been able to make her cum. Later when he came at home, I asked him to fuck Susie in doggy and she cummed immediately. (you'll be able to find a lot of pics of them fucking in that position in this site). I know the fact he is black turns my wife on like crazy.