bleach vs. color removers
In most beauty supply stores you can find products labeled for removing hair color. These products lift the color that you have previously applied to your hair to generally the lightest stage it was at prior to applying the dye.
Since most commercial dyes work by bleaching your hair slightly while depositing pigment the end result may be hair that is lighter than you expected.
color removers are considered a bleach alternative, but it should be noted that most of these do actually contain bleach.
The process by which they strip synthetic color from your hair is less damaging than using bleach. So I recommend that if you have dyed your hair black, a color that is notoriously difficult to lift, that you use a color remover first. Then apply bleach to lighten to the level of blond you want.
You will almost always want to follow the color remover with a bleaching session, because the results are seldom pretty. So if you go this route please pick up bleaching supplies while you are purchasing these products. Or have a good hat or unibomber hoodie available to hide your hair.