Anyway, oddly enough, I understand the details of your argument, but I don't see the big picture that you're presenting.

Ah, then perhaps I'm more confused than I thought! I still haven't identified the source of my confusion, though.

So, would you agree that the question "how does consciousness work" is no different from "how does blood flow work" ? If not (as I suspect is the case), then what's the difference ?

Er... Yes and no. I agree that eventually we should be able to find an answer that sounds as reduced as an answer to "How does blood flow work?" does. But from where we currently stand, they seem to be really, incredibly fundamentally different questions - as long as you understand the question "How does consciousness work?" to be in the hard sense rather than in the easy one.

I think you get near to the crux of the matter in this statement:

Presumably, a sleepwalker's brain, or a robot's circuitry, or a zombie's... er... goo or whatever it is zombies have, would implement this functionality in different ways than normal human brains do; and we could tell whether the sleepwalker/robot/zombie implements this functionality or not by talking to them [...]

Yes, presumably that's the case, and eventually we'll nail that down. But from what we can currently tell, there doesn't seem to be even an in-principle plausible mechanism for adding qualia to a computer's way of processing things. A computer receives input, does some well-defined manipulations, and offers output. Where do qualia come into play? How is it we get the subjective impression of there being a "someone" who is "watching" what's going on in the Cartesian theater? The very concept is internally inconsistent (e.g., how does the homunculus experience?), but the point is the same: there doesn't seem to be any plausible way that we have currently thought of to get from neurons firing to qualia.

I guess the categorical difference is that when asking about blood flow, there's someone who experiences the question and the data and the subsequent answer; but when asking about consciousness, it's the very process of being able to understand the question in the first place that we're asking about. I'm not sure that's entirely equivalent to the hard problem, though.

You might find it helpful to read the Wikipedia page on the hard problem. That might help to explain some of the nuances better than I've been able to thus far. (In particular, it helps to point out that by "hard problem" I don't mean "a challenging problem" but rather "a problem whose potential to be answered even in theory seems in question.")

[...] (as you have pointed out in your thought experiment).

Again, I think that was Yvain.