I downloaded j2sdk-1_3_1-doc.zip from Sun. When I unzip it I get an error: docs/api/index-files/index-9.html bad CRC 7639d5ea (should be f9fee48b) I tried 3 of the mirrors and got the same result. It seems pretty obvious that this file does not matter. But 'emerge blackdown-jdk' fails because of this. Sun is alerted of the problem.
A workaround seems to be to unzip the file, zip it again and put the resulting zip in the original place.
Can't reproduce this problem. I emerge java-docs-1.3.1 just fine over here, after downloading the file from the Sun website.