You know, Sevda, you were lucky to get a black that discharged to white, your very first try. Many - most? - commercially dyed black garments on the market won't discharge all the way to white, but instead produce some peculiarly old-looking shade of burgundy or rust or brown, which can be interesting but makes it hard to overdye just the colors you want, or even just stay black.
Even when you find a garment that discharges perfectly, and buy more of it on a regular basis, all will go well for a number of months or even years, and then, suddenly, the manufacturer starts using a different dye, without telling you, and the discharging gives different effects than you are used to. So, this is just a warning not to invest in a truckload of some particularly pleasing black clothing blank, only to discover that the results change.
When you start dyeing your own stuff black, you'll find that there are several different black mixtures in the Procion MX line, but all are mixtures of different colors. Some of the dyes in the mixture discharge better than others, so the different mixtures produce different colors when you discharge them. They won't discharge to white. They tend to spread out into different colors when you apply the dye directly, which can be nice-looking in itself but is a major pain when what you want is BLACK.
If you want a true black that stays black when you do
LWI dyeing, or that discharges nearly to white, the best option is to use a specific fiber reactive dye from an entirely different line of dyes, the
Remazol or Vinyl Sulfone dyes. You can use it on the same garments as Procion MX dyes; there's no need to invest in the entire line of Remazol dyes when you just want a perfect black. You can buy this black dye from PRO Chemical & Dye as Liquid Reactive Black LR604, or from any supplier of Jacquard Red Label Dyes (I think Dharma might special order it for you) as Jacquard Red Label Silk Color Black B. (Even though they tend to sell it as a silk color, I use it mostly on cotton, with soda ash and a little extra heat.)
Paula