Naa, strings aren't dirty and the guitars are always kept clean.
Rust? Um, don't think so, but I'll look into it. LOL
I use D'Addarios, I've tried many different brands just for the sake of tone and always had this problem. (I did notice that Earnie Balls seem to last FOREVER, but I didn't like the tone . . . just a side note there.
Perhaps it's not carbon, looks and tastes like it, but I could be wrong. I know a couple of other people that get the same thing and I'm almost sure I read it's something to do with the acid in skin oil and/or sweat.
Some people will have this problem, others won't, simply body chemistry.
Oh well, doesn't hurt my playing, I suck anyway.
I'll just try to stop licking my fingers. Bad habit anyway.
But I'll take that over doing drugs or smoking any day. :P