I had the same problem with Blog Module's Archive Widget (Browse by Date) not working. So I followed the advice to try the latest two trunks http://www.silverstripe.org/assets/modules/trunk/blog-trunk-r83655.tar.gz and http://svn.silverstripe.com/open/modules/blog/trunk/ (83720). Same result with both: The Browse by Date function works fine now BUT the categories (Tag Cloud) does not work now (but did befor the update).
Second problem since that update: I use two blogs (two BlogHolder pages) in my page and the categories of both blogs seem to be mixed (meaning one blog reads the categories of the other blog).
Third problem since that update: While the Blog I set up first does show widgets in the sidebar, the second Blog does not. In the backend the blockholder page of the second blog does not accept any widget get draged and dropped into the "Widgets currently used" area.