How Do You Strip Black Hair Dye?

As a beautician of 32 years I recommend Highly to go to a colorist first. It costs but you will see the best results. There are several factors to be considered. The texture of your hair (fine, med or course) What type of color did you put in (permanent,demi, or semi permanent) what kind of shape your hair is in (healthy,strong but dry, dry and damaged.) semi permanent color only penatrates the top layer and fades quickly,demi is a little stronger and permanent can penetrate to the Medulla or almost to the center of the hair follicle If the latter happened, more than likely the color will not come out even with a bleach/soap cap mixture.  (especially if you want to go blonde) You can try using a mixture of 1 pack lemonade koolade unsweetened mixed in a bowl with Redken cleansing cream (it is a clarifying shampoo) using an application brush put on the sectioned off hair and work it in. Put a plastic bag on your head (you can wrap around plastic wrap) and get under a dryer or heat source for about 20 min. Take the bag off and work the mixture through your hair again. Rinse and shampoo as normal and condition. I have seen this formula take out Iron from well water. It may take more than one fresh application to work.
The other option you might try is go to your discount store and purchase a product called OOps! It is supposed to take out at least a level of color from too dark of hair.
The condition and texture of your hair will make a difference how much of the black will come out. Anything any stronger will damage your hair further and you need a professional to make sure this is done right or the results could be disastrous.