ooh, possibly - but probably not realistically.
in theory, if they are grouped around one of your commanders during a battle
1)they can be used for blood battle magic
2)i've heard that at the end of the battle, if they are near a mage, they can be "re-gemified"; but i haven't actually noticed this as I'm not in the habit of travelling around w/ loose blood slaves
the lesson is: move your captured slaves to a lab on a fairly periodic basis