The question: "is there anyway I can practice sucking off black cocks before I choke
The truth is no, of course you can use a bannana, or dildo to test how wide you can stretch your mouth and throat, with breating excercises, but nothing is like the real thing. All sissies and most cucks really want to suck cock and take it up their apussy. Deepthroating is a talent that has to be developed and I feel way over rated, even if Linda Lovelace does not agree. I must say again not all cucks are sissie, poor things, but even if you are putting us on, I think you are.
I wish to congratulate on the open lines of communication between your female and yourself. Have you considered approaching the most approacable of your "buddies" and explaining your situation, and asking him to allow you to practice on him, so you do not gag at the wrong time. You will find you have power over them, when you have their cocks in your mouth, or hot, tight ass.
You are or you are not, only you can decide!