Gentoo? Excellent.

one or two things before I tell you how to remove blackdown.
Because of portage's dependancy tree, you will more than likely get blackdown again later wether you want it or not. The gentoo devs are slightly biased towards blackdown, so they don't seem to include sun's java as an alternative.
However, it shouldn't matter once you get sun's java installed with no blackdown - Suns should be used by default.

open a console, then do the following:


su -
emerge -C blackdown-jre blackdown-jdk


This will REMOVE blackdown. If you want to make sure that's all it's going to remove, do emerge -Cp blackdown-jre blackdown-jdk first.

Once done, visit http://java.sun.com/j2se/1.4.2/download.html
and download the SDK.

Read the terms/conditions, accept or decline. If you accept, hit continue.
Then, scroll down a little if needed and download the Self extracting binary (Not the Rpm, and ensure it's for Linux).

Now, this part's tricky. The binary never actually works.
To try anyway, do this:


su -
chmod +x j2sdk-1_4_2_05-linux-i586.bin
./j2sdk-1_4_2_05-linux-i586.bin

Read the terms, choose yes or no.
If it doesn't work, you will see something such as:
tail: cannot open `+479' for reading: No such file or directory
after agreeing to the terms, and some stuff asking you to please not attempt to install this archive file (Which I am about to tell you how to do. :P)

Open the file with an editor.
Delete everything, until the last line you delete is:
exit 0

Then you should see ^?ELF^A^A^A^A. . .

Save the file as is, and then run it again.

Now, you should have a folder in the directory you ran it in: j2sdk1.4.2_05

from a console, do:
su -
mkdir /usr/java
mv j2sdk1.4.2_05 /usr/java


Now, theoretically, you should have sun's Java.

Good luck!