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