What was her reaction when you told her of your fantasy? My GF and I have been doing what you want to do for the last eight years. But, it was her suggestion we go to a swing club and see what it was like. And, she took the first step having sex with another guy the second time we went to the club. We have had fun swinging since. And, yes, as you can see from my album, she loves to play with BBCs now. And, I enjoy seeing, and hearing, her be pleasured by BBCs.
I have heard of lots of couples where the guy pushed the gal into having sex with another guy and there was a lot of drama afterwards. Suggest you take your time and let the interest builds until it is, in effect, her idea. Do some role playing with a large black dildo, watch some MFM movies where the visitor has a large cock, etc.