Yes, this is the most safe and rewarding way to go.
If she feels the initial excitement of the thought, then you can actively support her with IR porn, BBC dildos, etc. - not the other way around.
The level of eye-contact / flirting gives her the playground to accept the excitement.
Instead of an ungraceful "I want you to sleep with black men..." you could ask "Did you recognize this black gentleman over there? ... Do you know him? .... Ah, not? ... Obviously he is somehow attracted to you".
The thought grows with each situation, where she recognizes that handsome black men are attracted to her. It pushes her feeling of self-worth and gives an exciting reward.