Thanet Blog List Currently Closed
Thanet Blog List is currently not completely open. In fact it is likely to be closed to new entries by most people. The reason for this is that it uses an excellent anti-spam script called "bad behavior" which spots spam bots before they get started.
Bad behavior, however, has an update after a blacklist was deactivated and thus returning a false positive on most, if not all, IP addresses. Simply downloading the latest version from the site and copying the core files over your current set will complete the update and remove the issue.