I recently changed an account password by accident, and was wondering how to set it back to a blank password via the shell. I have root privileges. Please help :)
passwd USER doesn't accept blank passwords
You can press enter to make the new password blank and then may get an error and have to enter the old password if you are not root but once you authenticate (or are root already) the blank password will be written.
You can set the account password to no password using the Accounts Preference Pane in System Preferences.