Make it similar to sparkling holy water or communion biscuits and have them regain 5 focus per preserved blood vial. The recipe is not too easy to make, and 5 focus likely isn't too much to make it too powerful in combat.
Otherwise their use is very niche, and someone asking for an herbalist or alchemist to make preserved blood currently gets suspicious looks and immediately "are you a vampire"?
Unsure what else could be thematic for elementalists to also have a source of focus to consume to regain it, but perhaps along with that, they could get some sort of "Elemental essence vial"