Basically, a vanity plate can't have a number sequence that falls into any of the pre-numbered sequences of any of the other plate types in order to avoid duplicate plate numbers being issued. I'm not positive, but I'm pretty sure that the "BLY###" plates were issued as standard University of Alabama plates (I have a "BMC###" in my collection, which would be only a few thousand after the "BLY" series) so somebody has probably been issued that number already. If you have any friends in law enforcement they can run that plate number and see. My dad tried to get "F250" on a plate several years ago but they wouldn't let him have it because the firefighter plates all begin with "F".
As for you getting the standard Madison county plates, you can thank people like the ACLU. Apparently it would infringe on somebody's fragile little snowflake rights for a state to issue a plate that has the word "God" on it as a standard issue plate, so the only way to get around it was to make the "God Bless America" plates optional. You actually have to request them if you want one, or else it could be misconstrued as the state forcing religion onto you. It just so happens that they're a "no cost" optional plate so you don't have to pay the additional $50 that you do for the other optionals.