Recently something changed in the format of Blogger blogs such that MarsEdit’s method of “auto-configuring” is now failing. This does not affect existing configurations in MarsEdit, but any new Blogger blog added to MarsEdit will fail to connect with a cryptic “Invalid Blog ID” error.
I’ve added a workaround of the problem to the Red Sweater Forums. The long and short of it is the “API Endpoint URL” and “Blog ID” fields in MarsEdit’s configuration need to be manually corrected.
I am working on a permanent fix for the next update to MarsEdit.