I was sitting in a second seat next to a window, aft of the wing, on the starboard side. our flight had begun its landing approach/descent thus my estimate of 20,000ft is based on the approximate duration of descent. i looked out the window, something i seldom do, and i noticed what i assumed was an shiny aircraft. it struck me as curious. because it appeared significantly closer than any other aircraft seen on any previous flight. as i continued to look at it, i then noticed it was trailing black smoke. by that i don't mean black smoke that appears as grey engine exhaust. i mean dense , black, opaque smoke being left as a black trail behind the craft. the craft had just begun emitting the trail when i noticed it because although it continued the black emission the entire time i saw it, there was only a very short trail when i first viewed it. as i looked at the craft, i realized that i really couldn't say with certainty that it was in fact a plane i had just assumed that it was on first glance. when i first saw it, the craft was below my plane and was quickly climbing at what i guess to be at least a 45 degree angle. because it was climbing at such an acute angle, i was able to have a better idea about its speed as compared to what i might have guessed if we were passing in opposite directions at the same altitude. i can't say with certainty that it was flying at an unimaginable speed but it was an extremely fast climb. concerning the crafts color, because i expected to see a plane i thought it had a silvery surface that was reflecting light but all i can say with certainty was that it was "bright white". i am certain the pilots my airplane also saw the craft.