Wordpress - create /blog subsite on WordPress multisite

Yes, you can I get a site at /blog running in MULTISITE, but that needs adding some code. That's because WordPress has a child-proof lock build in that you need to unlock first. This can be done by adding a hook function into the subdirectory_reserved_names filter that passes an array with names that are blocked as parameter. Remove the entries you'd like to use and return the filtered array.

Take care because this child-proof lock has been build so that the words 'page', 'comments', 'blog', 'files', 'feed' could not be used by default. /blog for example is reserved for the main site.

For more information please see ticket #13304, the related change that allows to override the reserved names was 14928

You can't.

That's part of the main site's permalink structure. There's no way to get around it. You can find more information here:

http://core.trac.wordpress.org/ticket/13527