I know that japanese maples don't really thrive in zones 8 and above. There are exceptions, of course, but they need to go dormant over the winter(40 or lower for a couple of months) and they can't do that in our warmer areas.
Is it in a pot or in-ground? In-ground is much better.
I would move it to a more shaded area...a couple of hours of morning sun or dappled shade for the day.

Here is a link to the black leaf problem you're having. Doesn't sound good, I'm afraid.

Here is a link that might be useful: Bacteria causes black leaves

I think if I were inclined, I would plant a dwarf variety in a large pot. Then I could move it around as needed. Maples prefer cooler climates than along the gulf coast.