Long time Lurker... New member... thought I'd share my review on stumbleblast:
First off, the OP's site does exactly what it says it does....
I found this site last week and decided to try it out. Purchased the 1 url package and submitted a new proxy site I own. Within 30 hours, my adsense revenue jumped from my daily avg. of $2 to $17.

I can say I was pleased with the results. After 48 hours my site had over 200 google listings, and was featured on Stumbleupon on the proxy keyword... the featured listing lasted about 1.5 days (more on this later)...
So later I purchsed the 20 URL package to try this on some client's sites who are paying me for SEO. I submitted a Real Estate site this time, but used only 100 initial stumbles on two different deep links. Again, similar results, within 2 days, the site was all over google, and the traffic increased significantly. Site was featured on one of they keywords I entered, but to make the featured listing last longer, I did a smaller stumble a week later, and noticed that after the 2nd stumble, the featured listing lasted considerably longer.
So there you have it... My secret sauce for getting the most out of stumbleupon. Seems StumbleUpon rewards sites that receive continual stumbles... Try an initial big burst of stumbles.. and then a much smaller weekly maintenance stumble.
I definately recommend this site...if used properly (and carefully), you can really boost your traffic. The 20 URL package works out to $0.014 per stumble... now thats cheap.