Reddit reviews Pack of 2 Rit Dye Laundry Treatment Color Remover

Run uniform through wash with a box of this :
Pack of 2 Rit Dye Laundry Treatment Color Remover Bundled by Maven Gifts https://www.amazon.com/dp/B0161C43RQ/ref=cm_sw_r_cp_api_NMz.Bb5R4140G

Here is a description of how I dyed my vest green. Even though you are doing black dye, what might help make it look black instead of very very very dark blue would be to use Rit Color Remover powder to pull out the blue dye first.

edit: In the photo album from the above post, you'll also find photos of my dying process.