All around Lir's Reach and in Farcrag Castle. If you can't find any, just pick up the common white chickens, wait until they respawn and I'm sure you will find some Black Dumpys then. As far as I know there are three places in Lir's Reach where you can find Black Dumpys (Highshore, Temple, Tavern) and you can also find three of them in Farcrag Castle. Hope this helps a bit. Happy chicken hunting!