Watch the film with her and ask her to fantasy being with a balck stud. Buy a black dilod and use that whiel watching (give a real name to the dildo). Tell her how you like the idea of being a cuckold to black. Good luck and let us know how it goes
You should keep mentioning that you know you are on the small side and mention that there are bigger ones out there -especially black - then buy her a big black dildo to emphasis teh point
Like all things a place to start is communication. Discuss with her and debate the pros and cons about sex.
Talk about racism and gently encourage her. This will take time so easy does it