Dying hair blonde from medium brown?

NO NO NO. Always bleach your hair before going to a lighter color. Sorry. Trust me, it'll turn out out awful if you don't. And don't mix it with the dye, the color won't be the one you want anymore.

Do not do that. At times, you can add a few drops of bleach to a color to give it a "boost" as my stylist said, but absolutely do NOT mix your bleach to attempt to get your hair blonde. That is a very bad idea. You have to separate the two or you will be very sorry.

Also you do not have to bleach your hair unless it has color on it. Color qualifies even if you dyed it a year ago - the color is still on your hair. But if it's been years and years or you think you've cut all the colored hair off, you could just dye your hair. You'll just have to use a higher developer (like 30 or even 40 if you're going extremely blonde). If you have colored your hair, then yes, you have to bleach in order to get to blonde.

Simply bleaching your hair will indeed turn your hair blond. The only reason you would need to use a blond dye kit afterwords would be if you didn't like the particular shade of blond your hair turned after bleaching. If you do decide to dye your hair a different shade of blond after seeing how your bleach-job turns out, I'd recommend giving your hair a break in between so you don't damage it (not sure how long, a couple weeks plus, at least). And remember to buy a shampoo/conditioner that's designed to take care of bleached/dyed hair.

The bleach will over-power the color. If your hair is pretty light, you MIGHT be able to just dye it blonde. But I would personally stick to the two-step bleach then dye or get it done at the salon.