When at last poor little Bilbo . . . poked his head out of the leaves he was nearly blinded . . . The sun was shining brightly . . . he saw all round him a sea of dark green ruffled here and there by the breeze. And there were hundreds [of] butterflies. I expect they were a kind of ‘purple emperor’, but they were dark dark velvety black without any markings at all. [-Tolkien]
Not only are purple emperors (Apatura iris) quite real, but their preferred habitat is the upper canopy of mature oak forests. Tolkien even gets the time of year right when they can be seen in the greatest numbers (late summer), and may have known about the occasional rare dark (melanic) specimens, an aberration known as Apature iris ab. iole.
[Rateliff commentary, “The History of The Hobbit”, Phase 2, VIII “Mirkwood”, ii “Butterflies”]