When package.mask is in /usr/portage/profiles/ then inverting a repository mask from /usr/portage/profiles/arch/x86 fails to work with paludis. But moving /usr/portage/profiles/package.mask to /usr/portage/profiles/base/package.mask allows the mask to be inverted and thus unmasked with paludis. This does change it from a repository mask to a profile mask.
*** This bug has been marked as a duplicate of bug 336692 ***