Supposedly if you stock pure strain BG into a pond with HBG eventually the pure bluegill become the dominant panfish of the two types.
I stocked adult HBG and BG 5 years ago-not really a long term study, but at this point the BG are dramatically more prevalent, in what appears to be pure form, than the HBG. I still catch some fish that appear to be fairly pure [if such can be used to describe a hybrid] HBG in different year classes, but very few compared to the BG. I haven't caught any that appear to be reverting to predominantly greenie phenotypes.
At least based on my experience, I think that if you have a good predator population, coupled with a good breeding BG population, you'll be OK.
