I know you’ve resolved it using .htaccess, but this is generally something you can amend in the site settings in your Network admin. Go to Sites, click Edit, and then scroll down to find the permalink structure which probably look like this: /blog/%posttitle%.
Just remove the ‘/blog’ part, click save and you’ll probably find it’s ok.
I did read somewhere that re-saving your permalinks over-rides this settings so you need to go back into the site via network admin to remove /blog again.
Hope that helps!