“BP seemed to move it into /blog/ subdir.”
BuddyPress did not move anything in your installation, even If you defined BP elsewhere than main blog,
Backup database, deactivate BP and BP-dependent plugins, then go to your database and delete around 15 tables with : yourprefix_bp_componentname
If you need to move WP to root, https://codex.wordpress.org/Moving_WordPress – as always, back up your database before attempting the move
So is that a default off MU to install in a /blog/ subdir ?
“So is that a default off MU to install in a /blog/ subdir”
No, you might have unpacked MU to the /blog/ subdir. I upload WPMU files and folders in root.
I didn’t create that /blog/ subdir, and I know I installed MU in the root, so trying to figure what happened
I’m sure I saw sth in the BP docs about it moving stuff to /blogs
To clarify, if I type in my domain the blog is there ok, but if I click on a post title I get domain.com/blog/posttitle
I originally set permalinks to custom /%postname%; now there is the prefix /blog outside the editable box.- same for catgory and tag base
I got it! I found the menu to edit permalink structure in MU – it’s not in the settings menu, but in Super Admin – Sites – Edit
I know I didn’t change it, so something I installed must have done it, if not BP then possibly the category mapping plugin
“so something I installed must have done it, if not BP “
I assure you, definitely not BP 
