When using the Sitewide Privacy plugin, setting a blog to private doesn't prevent content (including excerpts) from appearing in the activity stream and Recent Global Posts widget.
In the case of the Recent Global Posts widget, at least, content published while a blog is set to private will not appear. However, content published before the blog was made private does not disappear once the blog is made private. In the case of the activity stream, the Sitewide Privacy settings seem to have no effect on whether or not the content appears.
In an older thread on the activity stream issue (https://premium.wpmudev.org/forums/topic/sitewide-privacy-options#post-20389), it was suggested that the issue had been fixed. That does not appear to be the case or, if it was, the plugin and BuddyPress seem to be out of synch again. Any suggestions?