Maybe you've seen some type of "ash" on menus. I've seen it in a few cookbooks, mostly from the types of places that do tasting menus, and the general preparation is: put a fruit/vegetable/plant in a hot oven for a long time until it's thoroughly blackened, grind it to a fine powder, and do whatever you want with it from there.
I tried this the other day with the pulp from juiced blood oranges and ended up with a fine, smoky powder with only mild bitterness and a fruity citrus thing in the background. It's pretty unique and I'd like to actually use it in something, but I also remember reading that blackening things isn't really healthy. Topics like "does x cause cancer???" are kind of risky ground on a forum but I am curious if there are any clear-cut health risks to eating this. If not I see no reason why you couldn't use it, say, as a more interesting substitute for squid ink.