“The locks will work for about a day with a 15 amp fuse” – Sounds like something intermittent.
“it looks like a fire cracker went off inside- all black” – Sounds like a short circuit to ground. (And, as Wha Who said, that would blow a 20A fuse, too.)
“and then I’ll just notice they no longer work” – If the fuse blows without your operating the locks (i.e., it happens all by itself) then the problem is on the “hot” side of the wiring, between the fuse and the switch(es) or solenoid(s), whichever is connected to the fuse.
Good luck tracing this down. (I hate intermittents.)