Several popular packages require virtual/jdk or virtual/jre to be installed. Examples are swig and mozilla. For some reason only blackdown JRE and JDK provide this. SUN JDK ( and that is the original java after all ) provides virtual/jdk-1.4 and ibm-jdk provides virtual/jdk-1.3-r1 or something like that. Again, this would not be a problem if portage would look at these as at regular dependencies, but what seems to happen that not finding a literal "virtual/jdk" string portage attempts to install blackdown packages. BTW mozilla crashes much more frequently with blackdown than with SUN.
This has been fixed; all JRE/JDKs now provide virtual/java-scheme-2 to signify that they follow the latest JRE/JDK guidelines (ie, they can be switched between, using the java-config command). Remerge your desired JDK/JREs