You need a line for unstable containing non-free in your sources.list to get an official Debian package(s) of the Java otherwise you would need to search either for the Blackdown java sources or one of the guides to installing java on Debian using the Sun packages.
Code:
apt-cache policy sun-java5-jdk
sun-java5-jdk:
Installed: (none)
Candidate: 1.5.0-08-1
Version table:
1.5.0-08-1 0
600 http://ftp2.de.debian.org sid/non-free Packages