Per Magelo, while they drop at a less than 1% rate from most gnolls in Blackburrow, they drop at a 12% rate from a gnoll arcanist and the gnoll high shaman. That may not help much, but if you concentrate on those mobs, it should go a little faster.
Blackburrow gnoll skins are naturally rare drops. One could wipe out Blackburrow three times over and not see a single one, and to put it into a brewing recipe for just one drink result, is just sad. Prime example of how developers just don't know the game they work on.
I hear you loud and clear. I slaughtered all of Blackburrow over and over this past weekend and found only one skin. I need four to complete a side quest for the Rogue Epic 1.0. Crazy rare........