What about
black cumin? Have you ever cooked anything of/with black cumin flour? I might seem crazy, but now, if i make soup, i put tons of black cumin flour into it. The whole huge pot of soup becomes a black substance with quite a specific smell and taste.

It doesn't add so much density as flaxseed flour, so i also add flaxseed flour to it sometimes. I don't know if any sane person would make a scary black "mesivo" of his soup, but i do it willingly and i like it (especially, because it helps my digestion, +it's very satiating). (Plus, they say, it's very beneficial, and blah, blah, blah). I often add cayenne salsa to a plate of this soup (which is mostly made of beans or grains, haha), so i don't feel the strange taste of cumin flour so distinctively. Btw, tonight i'm going to make this kind of black soup (the time has come,- i've run out of any soup a long time ago), but this time it will be a pea soup.