You might have to try the latest daily build of the blog module to get 2.4 compatibility. I don't think 0.3 will work and no new stable version has been released. Daily build link is on the blog download page or use subversion to checkout trunk.
I had the same problem and was able to resolve it using the change described at http://open.silverstripe.org/changeset/99407/modules/blog/trunk. Have a look, make the switch, rebuild db, refresh admin and the error should be fixed.