TORN discussion here.
And yes there are black butterflies.
Two days later... they were in a valley filled almost entirely with a mighty growth of oaks.
"Is there no end to this accursed forest?" said Thorin. "Somebody must climb a tree and see if he can get his head above the roof and have a look round...."
Of course "somebody" meant Bilbo....
In the end he poked his head above the roof of leaves and then he found spiders all right. But they were only small ones of ordinary size, and they were after the butterflies.... [He] saw all round him a sea of dark green, ruffled here and there by the breeze; and there were everywhere hundreds of butterflies. I expect they were a kind of 'purple emperor',1 a butterfly that loves the tops of oak-woods, but these were not purple at all, they were a dark dark velvety black without any markings to be seen.
more screenshots HERE.