Is there a Django based blog available on the repositories of Ubuntu?
No, there isn't. This is probably because there is still no clear one-best blogging engine, those that are good still recieve heavy development (so are better suited to a git/svn/other VCS) so you can update more frequently, and because you can write your own (with a lot of features) in 10 minutes.
You can see all the django packages either in the Software Centre or by typing:
apt-cache search django