I planted 3 aspens in full sun w/ lots of fertilizer 4 days ago & their leaves turned black on the 3rd day. ??

Your plant didn't need fertilizer; and you did the right thing by watering. Keep watering-build a berm about 2 to 3 ft around the tree and fill it twice a day with water for a couple weeks--but don't water to high where the trunk is. Don't think climate was your problem. Trees that are indigenous don't need fertilizer for the most part because they're adapted to that particular climatic, and eco-geographical zone.