Comment 11 for bug 927540

That's actually a good point Lukasz, one that I assume folks get by reading the
multipath guide first.

https://help.ubuntu.com/12.04/serverguide/multipath-devices.html

Whenever you use blacklists, in either lvm or multipath, you must update
the initramfs so those config files are copied to the ramdisk, otherwise you
can get a race that makes it look like your blacklist is having no effect.

# update-initramfs -u -k all

should do it.