In the line preceding the crackling and dragging blacks, the speaker notes that "She is used to this sort of thing." The antecedent of "she" is the moon, as it is personified in the prior stanza, and the line is an elaboration on the statement that, "The moon has nothing to be sad about." Which ends in a preposition. But I just wrote a fragment, so it'll be all right.
In any case, when I imagine the moon crackling and dragging, I imagine its orbit through space. Taking into consideration that the moon is [probably] lifeless, I can't imagine anything on the moon would be crackling and dragging. The "blacks" could perhaps be referencing the "edge" of the moon, which looks black in the night sky, but that seems like a disappointing and shallow interpretation to me. What else is black in the moon, though? Craters? "The man in the moon"? I used to think the moon looked like it had a picture of a wolf on a cliff howling at the moon in it. I don't see it anymore, which is disappointing.
The title would probably have to refer to more than merely the edges of the moon as perceived from Earth, however. It could tie into the line, "We have come so far, it is over." There, they (the feet?) reach a sort of metaphorical edge. It could be the edge of life, as many have already noted. Knowing as we do that Plath favored confessional poetry and also that this poem was one of the last she wrote prior to her suicide, this interpretation would make align well. Maybe the perfected dead body of the Greek woman is her twisted glamorization of death. Having read Ava's post, I do see now some oddly striking parallels between Plath's mindset and Nina's in Black Swan.
It reminded me of that song a little bit. That song almost became my eighth grade class's graduation song. It's a crazy world.