After emerging blackdown-jre and running java-config --set-system-vm=blackdown-jre-1.3.1, portage gave me errors after trying to emerge any package. It turns out that there were 2 lines in /etc/env.d/20java that ended in "=" (they were JDK_HOME= and JAVAC=). Removing these two lines solved my problem. The errors I was getting are reported in bug #1535: http://bugs.gentoo.org/show_bug.cgi?id=1535
Fixed in java-config 0.2.1