Well, I would say go to a salon too, but if someone gave me that advice, I'd say $$$$! lol. So if you're in the same boat, I can try to help. When you stripped your hair, you removed the color molecules, leaving the hair very porous and able to accept new dyes and color pigment molecules. Thats why you bleach before dying funky colors, it isn't just because it shows better on lighter hair, the bleach basically eats a bunch of holes in your hair, and then the new colors can bond and stay.
I have done the exact same thing you describe to my own hair before. The thing is, your roots haven't been through as much processing, so they aren't as porous, and will react to box type dyes basically as intended.
After stripping your hair below the roots, leaving open "holes" you basically put new hair dye. Your roots picked it up as it should, the rest clung to any brown molecules in the blonde dye you bought.
Before doing anything else to it, I would replenish and make use of pony tails/hats. To replenish, a filler to fix some of the hair will help. Keratin treatments are good, I like APHogee brand.
If it was me, I'd do a hair fixer treatment. ( http://www.amazon.com/Loreal-Hair-Fixer-6-applications/dp/B000CPJPT0/ref=sr_1_1?s=beauty&ie=UTF8&qid=1324482652&sr=1) Deep condition (whatever deep conditioner works best, usually something you have to leave in, I like Cholesterol type stuff for this kind of job.) Then keratin (http://www.amazon.com/Aphogee-Keratin-Green-Tea-Restructurizer/dp/B000142P12). Then another "filler" treatment (http://www.sallybeauty.com/Aphogree-treatment/SBS-670567,default,pd.html?cgid=Hair02 ;), and one more deep condition with whatever you chose. That should get the hair ok to work with again. Use everything according to directions if you choose to do this. You will also need stuff to recolor with. I still think its cheaper than the salon, but a lot of work on your part.
Use a neutral protein filler before using dye, that will fill in some of the holes to keep color from bonding to them. I still don't think you're going to be able to go platinum or anything at this time without pro help.
You could technically try stripping once more after giving the hair some time to heal, and then bleach the ever living hell out of it while using a filler, but you could possibly fry your hair beyond repair, and/or end up with the same results if you're unfamiliar with the chemistry. When I did this (about 5 times in a very short period), I ended up getting everything but the roots chopped off and starting fresh with a Jamie Lee Curtis hairstyle because my ends had chemically fried together and were the texture of linen cloth.
So in the end, you may have to just go to a salon, but if you want to try my "home remedy" have at it, I'm all for home experimentation with hair, especially when you know what elements you're working with. I see this is from November, so hopefullyyou found help by now, I just felt like sharing, as hair dye is a hobby of mine lol. So good luck to you or anyone else in the same predicament!