Your wife had BBC at College, so talk gently to her about whether she enjoyed it or not. This will help her to build up mind pictures of the possibility of it happening again. If she understands why you would like her to take a black lover, and that you will remain monogamous to her while she is polymorous, then she will ease into BBC again.
Don't push too hard, give her time to handle the "roll back of the wedding vows" to allow her the freedom to have fun dates with black studs of her choice.
Offer to buy her new clothes for her dating, and give her the freedom to decide how long each date will be. Don't put rules on her dating, just encourage her to go with the flow of ENJOYING HERSELF TO THE MAXIMUM.
She will reward you with better sex, providing you treat her well and put her pleasure on a pedestal ahead of your own.
Help your wife by taking on more of the household duties, to free her up so that she is "fresher" (less tired) and therefore has more energy to give to this extra sex.
Because you want her to go BBC, she must feel that she has as much time as she needs to "grow the relationship" with a Bull lover. This could sometimes mean, she can't spend as much time with you. She may want to stay with her Bull for 1-2 days at a time if she is having a good time. Show you love her by giving her THAT SPACE.
Initially, she will want personal privacy to be with her bull, and if she wants to bring him home for the night or weekend, then you should offer to vacate the marital bed and sleep the night in a spare bedroom. This demonstrates your desire for her to enjoy herself.
Good luck.