I'm not sure since which version this happens, but at least on 0.7* and 9999 building sys-fs/zfs causes a QA warning about the --with-blkid configure option being unknown. From the looks of it, it can just be dropped without any further changes.
I missed that bug while doing pull request and did not add Closes: tag It's fixed in https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=b976c04437daee44030ddd28f757a7df1ae5df45 Please close bug as time permits, thanks!
Thanks Georgy. I see the "- --with-blkid" change. Closing as Resolved.