Thanks for the replies. I put a couple of them to use today. My 2400 psi power washer had a long enough pressure hose that I didn't need to (nor would I have tried) to put the washer on the roof. It did a good job of washing the roof and getting rid of the black spots.
However, there were brown stains left where the black spots were. I then poured a little bleach and spread it around with a sponge mop and let it set for a while. Then a little more scrubbing with the sponge mob and a rinse off with the pressure washer and things were much, much cleaner.
This forum is a great resource-----------thanks again for the responses.