I did weight of darkness with a friend and found 1 blak kat? (typo in purpose :P)
Anywhere where large amounts of Kats can spawn. Candlestick Keeps, Scarlet Fortress's and Haunted Passages are your best bet. I been farming those levels on Tier 3 Ruby Sun Gate.
The mission "Occurrence at Owlite Keep" has plenty of kats :U
You can also try the mission "Return of Ur".
Anywhere where large amounts of Kats can spawn. Candlestick Keeps, Scarlet Fortress's and Haunted Passages are your best bet. I been farming those levels on Tier 3 Ruby Sun Gate.