you'd basically want to use twice the amount recommended in a pack of black dye for one shirt. if there's not enough black pigment, the shirt (or whatever garment) will have a dark purplish hue once it starts fading.
if it's just one pair of sweats, i found that the best way of dye-ing is to have a big pot of water on the stove, and that's where you would dye the item. (of course once u use the pot for dye-ing, u cant use it for cooking anymore). the hotter the water, the more the dye seemed to stay. just follow the instructions on the dye packet.
you'd also want to use a lot of salt or vinegar while dyeing because it helps "seal in the colour". once the sweats are done in the dye bath, you might need to iron them because the heat will also solidify and seal the pigments to the garment
hope that helps