https://imagizer.imageshack.us/v2/18...661/vBZmlD.png
Using that chart it shows to use one plum to hit black from kobold brown. Just stop after your first feather message and see if that works?
Sometimes you need to feed more than 10 of a fruit to get a message. If you don't get one the color will not change. I'd refer to the chart as well. It's been fairly accurate for myself and a few others. Some FC mates were having trouble, but once they followed the chart they got the results they wanted so it seems fairly solid.