blackdown-jdk and blackdown-jre are linked to libstdc++.so.5 in various binaries. gcc 3.4 does not provide libstdc++.so.5, but rather libstdc++.so.6. blackdown-jdk should therefore RDEPEND on <sys-devel-gcc-3.4.0.
Or alternatively Lv's libstdc++-v3-3.3.3 compatibility package; either is good.
i've fixed stuff like this by adding libstdc++-v3 to the amd64 system profile. not all archs will need the compat package, and i dont want to edit every single binary package in the tree. emerge libstdc++-v3, which will be in the system profile for your arch once it makes the transition to gcc 3.4