The BlackEmperor butterfly can be found aobve the forest canopy of Mirkwood forest. They seem to be a slightly corrupted version of the elusive Purple Emperor butterfly that also can be found high in the oak trees.
Bilbo encountered the BlackEmperor in the canopy of northeast Mirkwood. Bilbo had been sent by the dwarves to scout out how much further they had to travel to reach the edge of the forest, in their journey to Erebor. Unfortunately although Bilbo had climbed a very tall tree, that tree was at the bottom of a wide valley so Bilbo could not see the end of the forest.