I dyed my hair pretty much black 6-7 months ago...

Once you dye your hair, normal hair dye won't lighten it. That's because hair dye lightens natural color, not artificial color that was put in there from the dye.

To go back to blonde, you can either a) prelighten it with bleach then color or b) remove the artificial pigment with a hair color remover then color.

One I have used that I like is called Color Oops which can be found in drugstores and some beauty supply stores. There are others that I've heard are good such as Color Zap. The color remover will damage your hair less than bleach and it removes the artificial pigment not the natural pigment.

Black is hard to get out so your may need to do it twice. And afterwards, deep condition and stay away from ash tones when you color. Neutral tones would be a good safe bet.

Whatever you decide to do, good luck to you. Let us know how it turns out.