Black algae is not a good thing! I wonder how it got into your pond? did you add anything to the pond in the last few months? (ornaments or wood or new plants?). If it was me I would be scooping it out with a net and keep doing that regularly until it is gone. Do you have fish in the pond? is there filtration or main plants somewhere? you have an excess of plant nutrients which is why the black algae is going strongly and also the green algae that you have looks to be going pretty well. Plants in the pond or good filtration to control nutrients should fix the green algae and netting the black algae (and keep doing it when it comes back) will eventually get rid of it.