This page, as I understand it:
http://wordpress.stackexchange.com/questions/67840/where-does-the-media-library-live-in-the-database
seems to confirm what I wrote before. If wp shows in media library what is written in wp_posts and wp_postmeta, it seems certain that if you upload an image in blog 1 (=write its path, data etc) in the wp_posts, wp_postmeta tables of database 1, when you open the media library of blog 2 it won't display those images. Unless its own tables too have been somehow updated... Seems a job for wp-cli, if it is possible...