There's some rng involved so multiple people could use the same food and get different results. The chocobo color calculator says you need Valfruit × 1 and Cieldalaes Pineapple × 1. I would feed it the valfruit and if it says the feathers are changing, wait to see the result. If it's not soot black, feed it the pineapple and wait. If it still isn't soot black use the calculator again, view the feeding order at the bottom and stop/wait each time the new feathers growing message pops up.