If you get errors about com_err blocking on your Gentoo system then you are probably experiencing a portage bug.
I was trying to install nfs-utils and received this error (shown in part):
[blocks B ] <sys-fs/e2fsprogs-1.41 (is blocking sys-libs/e2fsprogs-libs-1.41.0) [blocks B ] sys-libs/ss (is blocking sys-libs/e2fsprogs-libs-1.41.0) [blocks B ] sys-libs/com_err (is blocking sys-libs/e2fsprogs-libs-1.41.0)
Some Google-ing led me to the following Gentoo bug and a solution. (See comment number 7)