Yep, you will find it in every machine pertty much. I think it is a carbon like build up from all the elctrical comings and goings in and under the machine. You will notice things like the flipper coils especially and the coil sleeves will be covered in it. If there are ramps under the playfield, good chance they will be black as well if they have never been cleaned.
I am sure that there is a much more technical term and explanation for it but main thing is, it's nothing to worry about, just something to clean.
