Some of the most popular code projects at codeplex.com are blog engines. Here's a few to see:
http://www.codeplex.com/blogengine - Apparently this is the most popular one if you measure by downloads and its had a lot more use experience than the others.
http://www.codeplex.com/oxite - This one will probably surpass BlogEngine in functionality and code quality based on the support for it.
http://www.codeplex.com/dnnblog - DotNetNuke seems to have the biggest ambitions with plenty of subsites and functionality planned. However its yet to get as big of a mindshare as BlogEngine or Oxite.