Deepwood grenn has the folowing numbers r25 g36 b28,
Soot black is r37 g35 b30
so you would Need to increase red and blue without decreasing green, so you probably should feed it Doma plume+Cieldas pineapple+Valfruit (the order is not important) one each then start again until feather message Comes.

Alternatively my Chart says that from Deepwood green you Need to use valfruit to get black; but we don't know how much your rgb values differ from the Standard Deepwood Nubers (they could be a bit above or below) so I would asume the first method with three fruits works better because with just valfruit you could also get to void blue again...
I hope that helps and you get your black choci