Would you not be just as far ahead to die the number in her in blond? If she's black. I may be misunderstanding what you mean. If my dog had black sides and ate off paint I would use blond dye on her.
Use bleach the same as you would on a spotted dog just put bleach on your stencil and number the dog. Let it work for about thirty or fourty five minutes and wash it off. U will have to do it a couple of times I like to do it a day apart. You have to wash them out if you don't and the hair is brittle the bleach may burn hair out.
What cole said will work but after every time you wash the bleach out you need to put some hair conditioner on it and leave it on the dog it will keep it from getting brittle
JUST A SUGGESTION. PAINT A WHITE NUMBER ON THE BLACK DOG , HOLD THE DOG UNTIL THE PAINT DRY,PUT A LOT OF BABY POWDER ON BOTH SIDES. THEN SPRAY BOTH SIDES WITH MOSQUITO SPRAY ,PUT FIELD TRIAL BLANKET ON DOG.BEFORE CAST,TAKE OFF BLANKET AND SPRAY LIBERAL AMOUNT OF MOSQUITO SPRAY ON DOG,THEN CAST DOG.THE MOSQUITO SPRAY WILL TASTE BAD ,ANDTHE DOG WILL NOT CHEW ON PAINT .THE SPRAY WILL NOT HURT THE DOG .