You should sit down and have a heart to heart discussion with her. Find out what she really wants. If she has no sexual desire for black men, and is just doing this for you, then what you are doing is slowly destroying your marriage by treating her disrespectfully as your pleasure toy, without regard for her feelings or dignity as a person.
On the other hand, if she has strong sexual attraction to black men (as did my late wife), then you have opened a door into a room that you may not be so eager to enter if you think deeply about it. Are you able to watch your wife being sexually pleasured by another man and enjoy her pleasure as much as she does, or will you not be able to handle it due to jealousy and self-doubt!
It pays to plan your way around that hornet's nest before you start poking it with a broom handle!