To get black and jet black shades you should dye grays with henna first, and then indigo. As soon as you rinse out the henna from your hair the next dye you should apply is indigo to get the best results.
Follow the steps for dyeing your gray hair with henna by following the instructions in the article: Red To Auburn Highlights Using Henna- Recipe and Instructions. After you are done, follow the steps outlined below to get your gray hair looking black again.
1 cup indigo powder
1/2 cup warm filtered water (might need more)
1 tsp salt (salt will enhance color)
Mix indigo powder, salt with water until it reaches the consistency of yogurt. It should be a paste–not runny. After rinsing out the henna hair dye, section your hair and add the indigo paste to your hair piling it on thick to all areas of your head, wear plastic gloves on your hands and use the necessary precautions before you dye your hair.
Put on shower cap and wrap towel around cap for increased warmth. Leave the paste on the hair for an hour or an hour and a half. Using an overhead dryer will increase the staining process but you should still leave plastic cap on for an hour. Rinse the paste out with cool water when your time is up.
Your hair will be black. Sometimes there will be a green tint to the hair, do not worry this will fade after a day or two.
Tip: Do not use indigo alone to dye your gray hair; your hair will have a purple blue tint if you do. If this is not a color that you want, then using henna first and then indigo will give you the black shade you desire.