How do you dye eggs black?

It depends on what fiber you're dyeing. Her eis hoe to dye cotton

Using Rit brand liquid black dye with a little Rit blue is the easiest way to
to dye anything black. The water temperature should be close to a boil
when dyeing anything black.

9 parts black to 1 part blue is a good ratio.

When dyeing silk and nylon use vinegar to help the dye adhere.

Instructions

1.Item to be dyed should be clean before dyeing
2.Fill pail with enough water that the item can move freely. (room temperature water for light color - hot water for dark colors)
3.Put item in water.
4.Soak item for 5 Minutes.
5.Mix desired color now while item is soaking using Direct or Rit Dyes.
6.Take item out of water after soaking for 5 minutes and place in plastic tub.
7.Add dye solution to water.
8.Put item in water.
9.Stir gently for 5 minutes making sure there are no twists.
10.While stirring add salt to water. 1 cup for light colors 2 to 3 cups for dark colors. (this does not have to be exact)
11.Stir for 15 minutes (light colors) - 30 minutes (dark colors). Darker colors will require hotter water.
12.Take item out water.
13.Rinse in cold water.
14.Dry small area of item with a blow dryer.
15.Look at item next to desired color target.
16.If you satisfied with the color skip to step 22, if not continue to next step.
17.Measure dyes for color adjustment.
18.Add dye to water.
19.Put item in water.
20.Stir every 1 to 2 minutes for 15 minutes.
21.Repeat from step 12 until you're satisfied with the color. If you are satisfied continue to next step.
22.Rinse item in cold water until water is clear.
23.Fill pot with room temperature water.
24.Add 2 ounces vinegar.
25.Add 2 ounces fix.
26.Put item in water for 5 minutes.
27.Do not rinse.
28.Extract in washing machine.
29.Dry in dryer.
30.Fluff every 5 to 10 minutes until dry. Hanging to dry may cause dye to migrate and become uneven.