Please check the following to avoid the HTTP Error : 503.
1.) Please check if there is enough free disk space for SearchBlox to operate.
2.) Delete the /searchblox server/webapps/searchblox/tempdocs folder before starting SearchBlox
3.) Delete the /searchblox server/webapps/searchblox/index folder before starting SearchBlox
After you access the SearchBlox console, please re-index the collections as the search index will be reset.