Go slow. An idea like that takes time for many women to come to grips with. Start by watching some porn with her that includes some black cocks. But don't go overboard to cuckold shots where the husband is eating the black's cum out of the wife's pussy. See what her reaction is and judge how fast to move forward towards your goal of showing videos of exaclty what you want her to do. Might take a few months.
If she is not into porn then bring up the idea of her fucking another guy during sex when she is horny. Tell her how hot she is and how you would love her to enjoy other men because she is sexy. Make it more about her than about you. Don't mention black guys yet. That can come later. She will likely want to start with a guy she is comfortable with and that might not be a black. Once you get her comfortable playing with others then you can suggest trying a BBC. But even then she might not be ready for you to lick his cum out of her. One step at a time. Eventually you will arrive at the point where you are enjoying BBC cum straight out of her dripping cunt and maybe getting fucked by BBC yourself!
