I was trying to figure out why some blocked items hadn't been blocked for the submitter of bug 724669, so I did some blocklist testing. I set up a new profile and tested it with Firefox 7.0.1 (I thought it might be a problem with old versions) and Firefox 10. In both cases, I was able to open the profile and install blocklisted add-ons. Looking at the profile folder and the blocklist preferences, it is clear that it wasn't downloaded initially, and probably wouldn't download until the next timer is triggered, which I believe is 24 hours later. The blocklist is really important, and I think it should be loaded as quickly as possible.
I think this is just bug 646639