I've seen a lot of trout that seemed very dark. I wouldn't have called them black but they were very close to black. The fish were all in high elevation streams, small streams, with a lot of shade. I just assumed they were taking on the coloration that would benefit them in their habitat. They were all small rainbow, less than 10". Could be the fish the OP saw was just an extra large one that hadn't seen much predation throughout it's life, maybe it wasn't afraid of you? If you had moved "quickly" to it or tried to grab it you'd likely have found out if it was dying. Just a thought.