My next step is to create another layer and fill it with black. I then zoom in a bit and use the standard eraser to take care of any spots which show up against the black that weren't evident with the blue.
You can hide the black layer at anytime by clicking the eye icon in the layers palette. I am giving a lot of detail in the tutorial, but I necessarily skip over some steps. If you have trouble with something and need more detailed instruction, e mail me and I will be happy to respond.