Going to POI when you suddenly remember you forgot a vial of blood? Have no fear, simply run north to the Plains of Havoc and find any kind of open container (mug, jug, etc.) and slay a minotaur or cyclops. Then use the container on the dead corpse. Instant "vial" of blood at your service.
Note: you can also find a non-decaying dragon corpse, which yields blood, in the Necromant House.