It would be fairly simple to clean the concrete with a scrub brush and some all-purpose cleaner, and then hose it off. Have you tried just spraying it with the garden hose on as high pressure as you can make the hose nozzle?

It is equally important to figure out why it is there. Is the black stuff appearing in shady areas? If it is mold, which it probably is, it's going to grow back over time.

Yes, it is in shady spots and we do think it is mold. Tried just hosing it and it didn't work.

Use a mixture of borax and laundry detergent, scrub on with a stiff brush on a handle. Let sit 15 minutes or so, scrub again and rinse!