By default WordPress will append “/blog” to the path in Settings > Permalinks for the root site
My Sites > Network Admin > Sites
For the root site, select Edit > Settings
Find “Permalink Structure”
The existing setting will be as set in the site’s normal Settings > Permalinks, but with the initial /blog specified too.
You can alter the /blog to be something else here.
Now go to the sites Settings > Permalinks and save it to cause wordpress to update htaccess
Feel free to comment if you can add help to this page or point out issues and solutions you have found. I do not provide support on this site, if you need help with a problem head over to stack overflow.