A post in the gentoo forums just helped me realize that blocks with a leading "<", e.g.
[blocks B ] <dev-libs/libxml2-2.7.7 ("<dev-libs/libxml2-2.7.7" is blocking sys-libs/zlib-1.2.4)
are quite easy to resolve by first upgrading the blocking package to a version newer than the specified one (2.7.7 in this case) and then proceeding with the original merge.
reinhardt@floyd ~ $ emerge -av1 libxml2 && emerge --update world
Writing this post helped me realize that escaping "<" can be important.