[Bill Douglas]

I had a wire going to the buss bar in the house that wasn't screwed on tightly enough and it had markings something like this. It had got hot enough to melt the plastic on the wire. When I talked to an electrican about it he said something along the lines of very bad and can lead to a house fire. Gulp.

Either way black soot shouldn't be there. For a start maybe (carefully) remove the screw and wire brush the area and do it up firmly, but remember there could be current going through it.