I believe you are referring to the 'Blueroom' forum which is intended for Teachers to register and share information.
That does not preclude the general public from downloading and using BlueJ within the stipulated license agreement. The actual license is a GNU General Public License as per the text available on the BlueJ site here:
http://www.bluej.org/about/license.html
The actual download link is:
http://www.bluej.org/download/download.html
Note that you will also need to download and install the JAVA Development kit (JDK) from:
http://www.oracle.com/technetwork/java/javase/downloads/jdk6downloads-1902814.html
There is also a link given on the BlueJ site that takes you to the JAVA site for the download.