I use Rit ProLine black dye and get great results. The biggest issue I’ve run into is media breakdown in the Blast. If you’re using glass media and haven’t changed it over in a while, glass particles could be getting embedded into your part and you’ll never be able to dye it with a deep black. If you suspect this could be your issue, I’d try manually removing powder from a part and dying it.
Another issue I run into with dying is the dye bath longevity. Less than a week after mixing, my dye bath is no good and will only produce blue-tinted parts. It seems like blue dyes are the least prone to oxidation, and no matter how much extra dye is added, I can’t get back to a balanced dye bath. Because of this I refresh my dye bath every Monday. The Rit ProLine is fairly cheap so it’s only ~$5 worth of dye to mix up a new bath. I also add about 1/2T of dye every time I have to replace evaporated/removed water from the bath.
Here’s a bit more information: