The flatblocks fixture needs to be loaded manually.
Which you can do by the following command:
python manage.py loaddata fixtures/flatblocks.json
The reason for this is mainly procedural in that we've anticipated non-developer admins updating the flatblocks (via the django-admin) on the server, and we wanted to lower our chances of accidentally overwriting the changes made by these admins (e.g. through a regular migration). This way, these admins make the changes on the server, let us know, and we update the fixture manually.
Hope that helps,