For 2 cup bicarb - a teaspoon of dye with 1-2tsp hot water Try a teaspoon of dye. It does take more because we are trying to get a solid black from a solid white base.
Then try about 1-2 tablespoons of hot water. Mix really well in a little cup until it’s dissolved. Add that into your soda. Also make sure you get any extras out of the little cup by adding some of that soda and scraping it out, every bit counts!
Then if you’re mixing by hand, mix super well, but also press the mix between your palms. Check for streaks, those are unmixed dye streaks. Keep mixing until you don’t see those. If you’re using a stand mixer just let it mix on medium for a good 5 minutes. Then let it dry completely. It will lighten as it dries so don’t be discouraged! It will go back to dark once you add your other ingredients.