It's sweet that you want to spare his feeling's but this is something he is hot for, so you can honestly say something like "knowing how exciting you think it would be to watch me doing it, makes me want to try it even more!" That way you shift the focus to the fantasy of it all and keep him involved.
You could also turn the tables and ask him just how hot does the thought of watching a big black cock like that stretch your pussy open make him? What would he do to get the chance to see it? Would he eat your pussy until you beg him to stop? Would he rim you? Let him do these things to show you how much he wants to see you ride that BBC.
Sexy things, turn it into a game while you determine just how serious he is about it.
Then you can tell him that if the thought of seeing you stuffed with a BBC makes him that horny for you, then you can't wait to do it
for him!
Now you can honestly tell him that the thought of riding that big black dick until you can't cum anymore
for him makes you hotter than anything.
It is
for him that you are willing to ride a huge black cock to multiple orgasms right?