It sounds a lot like this is more your fantasy and she is going along just to shut you up. If she really wants to cross the color barrier she most likely will with or without your consent. You mention that "I KNOW she wants much bigger". How do you know, has she told you? Before I joined the group I shared my bed with a number women that I met through various mean s. I learned real early on to talk to the female half on neutral ground. There is a big difference between thinking about an IR encounter and actually doing it.
Not all black males have big cocks and not all black males are expert fuckers. If you want your wife to have a good IR experience you need to find a black male that meets her needs, if they actually exist. The best and safest bet would be to join a swingers club. Everyone knows why they are there, they are very likely going to be very cognizant of STD's, and anyone you encounter is going to be pre-screened, clubs generally ban anyone that is any serious issues.
Some women simply cannot handle a big cock, however most women can handle more than they realize. From your post I assume that you wish to be present. Having you present at the initial meeting is probably a good idea from a safety standpoint.
So assuming you have found a well hung bull, and she wants to get fucked by him, are you ready really ready? Speaking from experience the first encounter with a new woman generally takes a while. Unless she has had a real big one she is going to be nervous. A skilled fucker is going to want to have her extremely aroused before he fucks her. Simply the fact that she is filled like she has never been heightens her reactions. If the bull is skilled and she is truly willing she will have multiple orgasms. If she really has a good time she is going to want to do it again and again. Are you sure you can handle that? One of the things that I can guarantee is that once you cross the line it will change your relationship with her forever. How it changes depends on the two of you.