Hello Jim...I am assuming you are the Jim of the JimBarb? I am in a similar situation with my wife right now; I too want to share/watch her take a long, fat, black cock. Until recently she has been extremely against anything involving black guys, however this past week she watched a gangbang porn involving three white guys, two black guys, and one lucky white slut. My wife became very excited by the porn, so now I am encouraging her to watch more and more IR porn. Also, I have ordered a "realistic" black dildo that I plan to introduce while her and I are having sex just to see what I can get her to try/do with it.
I would suggest that you try some of the same tactics and see if you can make any progress with your wife. I haven't shared MY actual fantasy with my wife yet, and although I'm beginning to suspect that she has an idea of what I'd like to see happen for her/us, I plan to only reveal my true plan after she begs me for the chance to go black. Best of luck to you!