The first question has to be has she shown any interest in or attraction to black guys? If yes, then that makes it easier. If no, then you are going to have to find out by talking to her.
The second question is does she already play with other guys? If not, dropping the bombshell out of the blue that you'd love to see some black guy with a huge cock fucking the ...... out of her might not go down so well!!
As WNY3somecple said porn can be a help or a hindrance here. A lot of the commercial BBC porn seems to involves some well hung black guy or guys fucking a white girl like they're trying to make sure she never walks again and there's nothing wrong with that, some of us find the rough sex element a real turn on in porn and in real lifebut if it's her first exposure to BBC porn it could scare the hell out of her.
But there is a lot of amateur BBC porn out there showing real wives, rather than porn stars, really enjoying IR sex. You know her better than anyone else so pick something you think she'll enjoy and if she does then that gives you a starting point.
But at the end of the day, if this is to work, she has to want this, not just because the idea turns you on but because it turns her on too xx