Depending on your natural hair color and texture, permanent, black hair dye can be either a nuisance or a monumental task to undertake. But despite that it's called "permanent," it isn't forever and can be erased from your hair within just a couple of hours. To be successful, you'll have to use a strong laundry detergent that has a good reputation for removing stains, and a leave-in conditioner that can help your hair recover from the chemicals in the detergent.
Wash your hair as you normally would, but instead of shampoo, use a strong, liquid laundry detergent. Powder detergent works also, but is more difficult to rise out. Avoid cheaper, store brand detergents and use a well-known brand that a good reputation for getting stains out, like Tide, Gain and OxyClean. Also, you should avoid using a brand with bleach, unless it's color-safe bleach, or your hair could be lightened by the chemicals.
Rise your hair and dry it. Avoid using a hair dryer because the heated air it produces could damage the hair even more. Instead, air dry hair or if you have to use a hair dryer, place it on the cold air setting. Once your hair is dry, examine it thoroughly--front, back and each side--to see if all the hair dye has been removed.
Re-wash and rinse again if the detergent didn't quite do the trick the first time. If you still aren't satisfied with the results after the second washing and rinsing, dry your hair and let it rest for several hours.
Apply a leave-in conditioner to your hair to keep it from drying out and breaking. Use the same conditioner for at least a few days to aid in your hair's recovery from being stripped down by the harsh chemicals contained in the detergent.