I have an American Strong Ale fermented with black cherries that I would like to enter. Should it simply go in as 29A Fruit Beer with American Strong ale as the base style? The cherry is present but not prominent.
Yep. American strong guidelines don’t account for any cherry flavor at all.
If you struggle to taste the cherry at all, you should enter it as an American Strong.
Note: Since you used black cherries (a.k.a., sweet cherries), I imagine the cherry flavor would be very slight or even non-existent. If you really want cherry flavor, you need to use tart cherries (e.g., Montmorency), and a lot of them.
The cherry is definitely there. I fermented with 1/2 gallon of sweet black cherry juice that had a SG of 1.086.