Did another trawl of the search engines (Bing, Duck Duck Go and Google) for "text mud development blog" and added the resulting links to Planet Mud-Dev.
Ilyrias MUD:
Last activity: September 2006.
Summary: Most likely a dead blog for a failed commercial MUD. Very few posts, one of which is on NPC behaviour and scripting.
Sea of Souls:
Last activity: December 2009.
Summary: Most likely a dead blog, but the MUD behind it is still live. A Wheel of Time themed Smaug. Tens of posts tagged with development.
If you know the URL of a MUD or MUD developer that has interesting development related posts and isn't already present here, let me know. Similarly, if you think a blog on this feed should be removed, also let me know. Post in the comments for either of these, or any other thoughts.