Black Algae - what does it really look like?

So what does it look like? Any pics? I was reading Ben's thing on Black Algae and he made reference to "trip and fall" which makes it sound huge.

I just noticed a couple of small black spots on one of my steps while brushing the other day, and they don't come off. They are very tiny, but it still bugs me.

Is this black agae that I should be worried about, or something else?

Thanks!

I am interested in a response to this as well - so I am bumping this :D