It could be your proxies or your settings (to many connections).I have 800 keywords loaded with SB, 20 proxies, I tried to scrape with footprint "site:tumblr.com" and SB harvests nothing. The harvester runs and stops. ANy help on this would be appreciated. Thanks.
Why don't you just use Scrapebox Vanity Name Checker addon for finding available blogs ?
https://www.youtube.com/watch?v=rz4s2scQpsU