I’m having the same problem and I have since long used the page located at /blog/ as the base for Posts, but the posts themselves are not ending up under /blog/. They are still under /<category name>/<post name>
Should note that I have /%category%/%postname% as permalink structure. No matter what I change this to, it still does not end up under /blog/ like I want it to. I _could_ of course set the permalink structure to /blog/%category%/%postname%, but that will break the custom post type ‘portfolio’. So I’m a little baffled.
Could it have to do with the theme? I don’t know if a theme can even control the permalinks…?