We are an experience couple so we will give you an answer from our perspective. First off, this is something you need to really sit down and talk about together and sounds like the lines of communication between you are good. We started out many years ago in the swinging lifestyle and we've had a lot of fun together. We are each other's best friend and we can discuss anything. We view sex with other people as a recreational activity we both enjoy. We too and interest in BBC and now have a preference for it although we still play with white people as well. Over time I explored my bi side with her encouragement and learned I liked it. We currently have one steady Bull we see, not as often as any of us would like but life does sometimes get in the way.
As far as advise for a new couple just starting out I think the first thing you need to discuss is to both express what you want from the experience. Do you just want to watch her with a Bull or do you also want to participate? What are her expictations and what does she want? Hopefully, she's not just doing it to please you but it's something she also wants. Watch out for jealousy issues, it can really screw up a relationship, talk,talk,talk. Next, what type of personality are you wanting in the Bull you choose? Do you want him to be a take charge guy and set the pace? Do you want him to be a dominant guy or do you want a Bull who will work with you both to give everyone the pleasurable experience you all seek. Just how far are you willing to go, will you both suck his cock if he wants you to? Will she be seeing him separately or always with you present?
Another aspect you don't see discussed here very often is the Bull in NOT just a piece of meat for your self gratification. He has feelings, wants and desires so when you decide on who you're going to invite into you're bed have some consideration for him as well. When you all set some ground rules you'll find things will go a lot smoother.
Once you take the first step and actually do it, you'll find it gets a lot easier and you will developed a sense for what you want and like and can taylor it to fit your wants and desires. Talking about it and doing it are too different things as fantasy sometimes doesn't live up to the expictations evertime. Good luck and if you have any specific questions feel free to ask and we will try and answer your questions. Hopefully, you will have an enjoyable experience for everyone and you will be left wanting more.