I tried to download blackdown-jdk and i got the same error:
!!! ERROR dev/java/java-sdk-docs-1.3.1 failed.
!!! Function src-unpack, Line 17, Exitcode 0
!!! Please downloadj2sdk-1_3_1-doc.zip from http://java.sun.com/j2se/1.3/docs.html
and move it to /usr/portage/distfiles
Okay so I'm an idiot. I followed those directions. I had to emerge lynx so that I could actually get the doc, but now it's emerging away quite happily. Sorry for waisting your time