I had some new strings fitted two days ago and they soun really tinny and my fingers are really black after using them. Last time I changed strings this didn't happen. They were ernie ball super slinkys
Some strings come oiled up in the packet, this is to keep them fresh and prevent rust I think. Perhaps that is what has rubbed off?

Strings will usually sound very "bright" or tinny as you said after new ones are put on. This is just because their tone is better than the older ones, you'll get used to that.
Did you change gauges? I don't know about the black stuff, but it explains unusual "tinniness."
I was going to say that Super Slinkys, and in fact most strings, tend to sound pretty tinny originally, however my fingers have never been stained or coloured by them. I'll assume it's normal and just say wipe the strings down with a cloth before and after use. It's likely a chemical added to preserve the strings that sticks to your skin due to sweat.
...Stapling helium to penguins since 1949.
thanks guys, yeah the black stuff is weird, there's loads of it 2 all over my fingers. x