Wordpress Custom Permalink for Just Posts
You simply have to set /blog/%postname%/ as your permalinks structure, this will not change your pages permalinks.
And to keep your portfolio permalinks, you should set with_front to false when you register this post type.
'with_front' => boolShould the permastruct be prepended with the front base. (example: if your permalink structure is/blog/, then your links will be:false->/news/,true->/blog/news/). Defaults to true
EDIT 1 : You should probably flush Wordpress rewrite rules after that.
EDIT 2 : with_front param is a rewrite param :
'rewrite' => array('slug' => 'portfolio', 'with_front' => false),