by itself? it's an astringent, good for oily scalps, and a fungicide - there's nothing in it that would darken your hair that I can think of.

in general, unless the shampoo says it's color depositing, it's not going to change your color in any permanent way - and even that takes a bit of time to build up on the hair shaft.

what might 'darken' it some would be blue or violet tints, cooling out the reds and blondes...but that would be hard to come by in an 'herbal' shampoo.

Many herbs are used as hair dyes.

Rosemary and sage are both herbs used to darken hair. The problem is that you have to use it often, and a lot of it, over a long period of time - it might take several weeks, or even months, to get the desired effect.

To make a herbal hair dye:
Simmer 60g of selected herb(s) in 1 litre of water for 20 minutes with the lid on the pan. Cool, strain, and pour the liquid through the hair, catching the rinse in a bowl. Repeat as many times as possible, using the same liquid over and over. Pat hair dry, using an old towel as some of the dye will come off as you dry. The more often you use these dyes, the stronger the colour will be. Commercial herbal teabags can be used, infused in 4 cups of water.
For more intense colour, make a paste by using 1 cup water, 30g selected herb(s) and kaolin powder. Wearing plastic gloves, rub well into the hair and scalp, cover the head with a hot towel and a plastic bag to retain heat, and leave for 20 minutes. Rinse clear.

TO DARKEN HAIR:
Use sage, or sage and rosemary leaves, or sage and dried raspberry leaves, or ivy berries.

You can also use ordinary black tea as a hair dye, using it in the same way as described above.

Walnut hulls with herbs are an extremely effective way to darken hair (and everything else). Always wear gloves when working with walnut hulls. Crush walnut hulls, cover with boiling water, and let them soak for 3-5 days.

Add 3 more cups of fresh water to the walnut water and simmer for 4 hours. If the water steams off add some more. Strain the liquid to get the hulls out. Reduce the volume of liquid to about a quarter of what you started with by boiling. Add one teaspoon cloves and one teaspoon cinnamon. Mix well. Now let it sit for a week or two in the fridge, It will get darker the longer it sits. Apply 15-20 times as a rinse to newly washed hair. DonÂt wash your hair right way after applying the rinse.

For a shampoo:
Mix 1 part liquid castille soap to 2 parts herbal infusion. Add 1 teaspoon glycerine per 200 ml shampoo, if desired - it facilitates mixing. To darken the hair, use rosemary or sage.