DocIII said:
I can see arguments that the null loses because he can't force the daemon to use its power to do what he wants.
Providing that the "power" in question actually is the daemon actively helping the wielder. The power could just as well be a side-effect of the daemon's presence in real space.
It all bottles down to metaphysical interpretation. One being that a bound daemon is actually a bound piece of the warp, trapped in real-space (the bindings mak:ing it impossible for the "thing" to escape back to the warp, and doing anything else that its creator didnt intend for it to do). The power is therefore steming from its own being (making it its own "battery" of sorts). Sure a very localized and constrained power (in comparison to a free daemon that still maintains its connection to the warp), but a power nontheless. And that power being bound within an object (a weapon) suffuses the object with unnatural side-effects (like being harder to destroy, having an edge sharp as sin, making the edge burn with pink fire etc.).
With this interpretation the daemon weapon could work in the hands of a blank.
Another interpretation is that the damon within a daemon weapon is constantly connected to the warp but restrained from escaping back to it in some way. Therefore the weapon gains its powers because the bound daemon within effectively makes it a lightning rod for warp energy, and that energy gets unleashed upon the victims the weapon is turned against.
With this interpretation, the damon weapon would not work when a blank is present, because the blank nullifies the powerflow from the warp to the weapon. (no juice, no twinkling lights).
Basically it is up to the Gm to decide what take he has on metaphysical mattes in the WH40K universe. As there is no official material on the subject. I personally go with the first interpretation. Partly because its cooler and more interesting, and partly because its goes in lieu with another RPG i've played, where you can create magical artifacts powered by: (yep you've guessed it) bound daemons! Those daemons worked as their own infinite power source, and the whole system concering artifact creation maked a whole lot of sense (slightly inspired from real-world physics and electronics), when comparing it to WH40K fluff (where there are no explanations what so ever). But thats my individual take on it. I can give arguments for them, but it will never be the "right" or "wrong" answer (until an errata shows up that is)...