Start with mainstream non-porn movies like Mountains Between Us, Nymphomaniac (2013), A Haunted House (2017), Jessica Jones Tv series, House of Cards, Runaways etc.
All of these and many others include some nice interracial scenes which you can use to get your gf used to it. Once you manage to naturalize it through enough exposure, start with light interracial softcore porn. Try hegreart videos (which I have posted) or some other romantic, artistic softcore
Discuss the symbolism and other issues concerning black people with her.
After that, start introducing a black dildo, preferably larger than your penis. If you're lucky, she will be prepared for blacked.com videos etc.
Then it's just a matter of time till the inner desire to try the real deal develops in her