I'm trying to install a package that is being blocked by a package that
portage claims is not even installed:

catherine ~ # emerge -av =dev-util/gtk-doc-am-1.10-r1

These are the packages that would be merged, in order:

Calculating dependencies... done!
[ebuild N   ] dev-util/gtk-doc-am-1.10-r1 395 kB
[blocks B   ] dev-util/gtk-doc-am (is blocking
dev-util/gtk-doc-1.8-r2)

Total: 1 package (1 new, 1 block), Size of downloads: 395 kB

!!! Error: The above package list contains packages which cannot be
installed
!!!     at the same time on the same system.

For more information about Blocked Packages, please refer to the
following
section of the Gentoo Linux x86 Handbook (architecture is irrelevant):

http://www.gentoo.org/doc/en/handbook/handbook-x86.xml?full=1#blocked

catherine ~ # emerge -pv dev-util/gtk-doc-am

These are the packages that would be merged, in order:

Calculating dependencies... done!
[ebuild N   ] dev-util/gtk-doc-am-1.10-r1 395 kB
[blocks B   ] dev-util/gtk-doc-am (is blocking
dev-util/gtk-doc-1.8-r2)

Total: 1 package (1 new, 1 block), Size of downloads: 395 kB

What should I do?

--
gentoo-user@(protected)