One method is to begin to frequently bring home videos that exclusively have interracial content in them. If you want to be successful at this manipulation don't start showing her an aggressive group of black guys gang-fucking a white slut! You should work your way towards that though, over a period of several weeks, since it will give you a bench mark of how far she is turned on by the idea, and also depend on whether your ultimate goal is turning her into a total slut for black dicks. But what ever your boundary is, you need to start off slowly and remember to approach the situation in a jovial fashion. Joking her along about "How about having a black guy then" and seeing what her reactions are etc.
When you first begin and you're watching an interracial scene in a video, it's very important to let her know how much it turns you on, not by telling her, but by showing her! Assuming that as you watch the videos, you'll make love to her by kissing and caressing her, slowly, make sure her pussy gets wet as you play with her. Then begin to eat her pussy more aggressively as interracial scenes are showing.
It would be a good idea at this point or earlier if you think she's ready for it, that you introduce a large, thick black dildo/vibrator into the love making, so she gets to feel what big black dicks are like. But for Christ sake, when penetrating it first off, do it slowly and carefully, maybe lubricating it with saliva or lube jell. You don't want to put her off the size, because this could go against you in the later stage of actually suggesting she try black!
It's important that when you use the black dildo/vibrator you do everything to see that her pleasure is increased, including bringing her to multiple orgasms with the dildo, your cock and your mouth, until she can't stand it and pushes you away!
The next step is a little more difficult, but by this point you should have found a way to verbalize how hot it makes you feel to see the white sluts in the videos with black dicks. Then as you're making love and using the black dildo on her, you should let her know that you've fantasized of seeing her in that situation with a real black dick. Try and say it in a 'matter of fact' style. Encouraging her to use the big black dildo/vibrator when you're not around might help reinforce the fantasy.
When out play a game, ask her to rate the attarctiveness of black guys, how good they are in bed, their cock size, etc. Let her know it's a bit of fun but it's a real turn on for you.
Good luck and let us know how it goes