Thanks for the picture. She looks like one of these white ladies who needs to be gangbanged. I would give you the advice to sell her for one weekend to a black cock gang. They will make her getting used to BBC.
Just be honest with her. I shared this idea with my wife and she actually responded well. She now watches interracial porn by herself, and when we fuck. She even watched a gangbang and really liked it. She has a massive black dildo in the mail now. Pretty soon she'll take the real thing.....just gotta find the right guy. Like I said be honest with her. Sit her down and talk. I started out by telling her that I had a dream that she cheated on my with a black man and that it actually made me horny. Once you get over this first bump, its easy. The worst part is getting the courage to tell her. Honesty is always the way to go....otherwise you may never see your dream come true.