We found 2 Reddit comments about Pack of 2 Rit Dye Laundry Treatment Color Remover. Here are the top ones, ranked by their Reddit score.
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.