[ebuild     U ] sys-fs/udev-118-r3 [109-r1]
[blocks B     ] <sys-fs/device-mapper-1.02.19-r1 (is blocking sys-fs/udev-118-r3)

 * 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