I read the Gentoo Printing Guide and I noticed I hadn't emerged the hplip driver with the ppds USE flag. So to get this working I did:

emerge -C hplip
echo net-print/hplip ppds >> /etc/portage/package.use
emerge -av hplip

and then I setup a new priter and it's now working well on either 300 or 600dpi

This is a lesson for me to RTFM from time to time

Well, I am not sure, but it seems some printing problems can be caused by a program from which you are pringing so trying a different program may help.

For example it is different when I print images from gthumb than from say Inkscape. If it'll even work it'll probably print it a bit differently.

It may be that another program would not print a black page, but it might just be that the program you are using currently is misconfigured somewhere (page setup or something).

Good luck

Oh didn't see your reply.. Anyway, disregard my previous post. I'm glad you found the solution afterall. Manuals, yes.

It's nice to see questions being answered promptly, even if it isn't the solution; it shows that there is an active community here

Hehe.. well if I see I can say anything related and potentially helpful I tend to say it, since we are just beginning and need to leave exactly that impression: an active community.

Still though, just that you guys are already asking questions here helps alot. Someone already here might just have an answer.

Cheers
Daniel