I think it depends wether your gf has been with BBC yet, what message she wants to send or if she wants/needs to keep it secret or not.
If its just for you and black guys to see, then above her privates or ass probably be best and she can be a bit more daring and obvious.
If its going to be on more public show, then you might want it to be more subtle and less obvious.
Also depends on a girls circumstances. Im in my 40s, married and not too botheted about other peoples opinions. If a girl is young and single, she may want it to be more subtle.
My own preference is the QoS. Mines a more obvious QoS but you can do clever designs with the QoS hidden within. X