I used to vape an Apple Pie juice that darkened in the tank. It was a dark color anyway, but in the protank, when I'd get down to the end, the little bit left would be almost black. I always just dumped it back in the container and shook it up to mix it with the rest before refilling. Not sure why exactly, but I guessed it was either just getting a little burnt from all that heat, or because whatever it was that made the juice so dark to begin with was not getting through the wicks at the same speed as the rest of the juice (it was always a bit thicker, too), or it was just settling at the bottom like coffee dregs. At any rate it didn't effect the flavor of the juice as far as I could tell, and after multiple times I realized it was always going to happen. So yeah, I think it's normal and nothing to worry about.