Ok this is what is called RED algae even though it looks black or dark green it is red under that.
I grows well in the presents of phosphates, which may get into your tank from either imported frozen blood worms or more likely your tape water.
There is little you can do other then removing the phosphate prier to use.
But here is a few things others have done.....
Get a black line flying fox they seem to eat it, nothing much else will.
Install a CO2 unit on your tank - your plants will grow so well you will be able to trim the older leaves that get the algae off and the tank will still look fab.
Sorry I can't be more helpful it's a tough little thing.