I'd guess it has more to do with the ammo you are shooting, powder, bullet specifically, than anything wrong with the gun. I know I'll end up with black finger from my Glock, sometimes, depending on what ammo I use.
Ok, I've been shooting some factory PPU ammo, maybe it is just dirty. I've never experienced a black finger like this from either of the 2 Glocks I use but I haven't shot PPU ammo before. I'll Grab some different ammo and see if it changes anything.