Question on dark/blackish carbon in tail pipe tips?

I've got an older ('86) chevy with a 350 v8, about 135k miles - it starts & runs great, no smoke ever visible at start-up, or while running on road. But... I do notice a significant amount of that blackish colored soot or carbon dust build-up in the end of the tailpipes (its got the small extensions on the tips in the rear, thats where I noticed the carbon/soot building up). I think I remembering hearing once that this is an indication of something, but I can't remember - maybe the engine running too rich, too lean, needing some kind of adjustment or part replaced, or maybe just a normal factor of having 130k miles on the engine & its nothing to worry about - can any mechanics or knowledgable motor folks out there tell me what it is an indication of? Thanks alot.