Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
That does sound like anaerobic areas.
Remove enough sand so the deepest is 2", or else use a gravel vac to turn over the sand at least once a month.
When you start working on the sand be very alert to the smell of H2S. It is toxic to the fish. You might want to remove them and do the job all at once, followed by a couple of big water changes to be sure it is all gone.
Cyanobacteria can also look pretty close to black, and almost oily, slimy, and could be growing between the glass and the sand. There is a surprising amount of light in that area. When you start to remove it you may notice an unpleasant odor, more like an earthy scent, but not quite that nice healthy soil smell.