Alright, I've managed to get of a picture of what's been frustrating me.
This is me, trying to erase the black pixels using the erase tool. The left mouse button is being held down. If I want to erase them, I need to either let go of the mouse button and click the spot again, or drag the mouse further than the spot I want to delete. Neither option is unworkable, of course, but it feels very unintuitive.