a) Blogs.dir only exists if
1) You're using Multisite
2) That Multisite instance was created BEFORE WordPress 3.5
If you started Multisite AFTER 3.5, then you're in /wp-content/uploads/sites/ instead
b) You're remembering wrong. There's only one wp_users set of tables
c) It's not easy. Just copying the tables over means you ALSO have to copy over all the users, in order to keep the username <-> ID balance in check, and then delete the users you don't need. And even then it may not work.
Why not just export and import via the WP importer? All your users would get new passwords, but that's not a horrible thing, security wise.