The first thing we need to rule out is it being a shadow. Notice that there are no noticeable shadows in any of the other craters. That's because the sun is at zenith. This means if you were standing on Mercury the sun is right overhead. This rules out the shadow theory.
If you are familiar with geology, you know that there are lots of different kinds of rocks and metals that are a dark black. The question is which one is it, and where did it come from.
It could be that whatever hit Mercury was made of this weird material. It could also be that
when the surface was impacted material from under the surface was exposed. Other dark rocks have also been discovered by the Messenger spacecraft. One of these is this ring of black
material. The first picture is interesting because it is the darkest. One of Messenger's science goals is discover what this black stuff is. Go Messenger!