When attempting to tie dye a dark coloured or black fabric, you cannot add new colour effectively so the process is referred to as reverse tie-dyeing. Reverse tie-dyeing is actually the removal of colour by bleach or a dye remover. This produces the same effect as traditional tie-dye but the uses of negative and positive space are reversed.
Soak the black leggings in cool water. Ring them out so they are damp but not dripping.
Lay the leggings out flat and begin to bunch sections -- the pattern is up to you. Wrap rubber bands around the sections you've created -- for a randomised look, try large and small bunches throughout the leggings. Wrap them tightly to keep the bleach/dye remover out. Put on your rubber or latex gloves.
Prepare a bleach and water solution -- approximately half bleach and half water though the strength of the solution depends on preference. You can also use a dye removal solution instead of bleach. If so, follow the directions on the packaging to ensure best results.
Dip the leggings into the bleach solution or dye remover. You can dip in sections or immerse the entire piece at once. Wait roughly 20 minutes and dip again if needed -- you'll be able to see the colours changing.
Soak the leggings in a bath of water and washing powder for a few minutes. Carefully remove rubber bands with scissors or a knife and place leggings in the bath again. Hold under running water to rinse.
Work with bleach in a well-ventilated area. Bleach can be dangerous if used improperly: wear gloves and do not inhale.