Hi Bliss, great that you're getting in to this lifestyle at your age. Can understand how exciting it must have been to learn that your wife has had previous black lover.
I think previous comments from rkccuckold are right, probably in your shock (or excitement) your wife misunderstood your feelings and so is being defensive over her past. So you need to undo this before you can proceed. Instead of focussing on penis size etc, I think you should tell her how proud you are of her that she is not prejudiced, not influenced by the taboos in your country. Tell her that you think it's pity that people have such negative views and that believe its good when IR relationships are more normal and actually you find it quite attractive yourself. From this perhaps she will open up a little and you can learn how they met, where they went on dates, how others viewed them etc. Once she is comfortable to talk openly and freely, then you can slowly develop situation. If handled carefully, I think you have great chance to adopt cuckold way of life.
There is also another great chance in that you can encourage her to communicate with this guy on FB to show her that you are not jealous. Perhaps in time you can connect with directly on FB? I suspect he will be only too willing to reveal more personal details of their relationship once he understands your interest.
Good luck!