According to the java documentation on the gentoo.org website, the blackdown-jdk should be listed when running 'java-config --list-available-vms'. This is not the case. Apparently the ebuild does not place anything in /etc/env.d/java as I believe it is supposed to (not too sure about that though). As a consequence of this it is not possible to set the system-vm or user-vm to blackdown-jdk with the java-config tool. Furthermore this makes it impossible to install openoffice from source, as it requires the blackdown-jdk.
This is a bug in Portage; if you merge the jdk more than once, the files will be removed. Carpaski, any progress on this one ?
This is fixed in the updated java.eclass