It seems to me that the very act of going into that blog page to make any edits will somehow cause the thing to break;
Look at this - the source code says it should all work and on line 30 we have this:
body class="blog-home page-template-default"
which seems to be right.
And yet it is broken. This happened to me on Pulse 5.03 so its not a new thing. Cannot remember how I fixed it before, I think I had to eventually nuke and pave.