obsessive correction of small, largely irrelevant detail (while upvoting in general agreement): what you subtract (the constant shift, which actually does have some structure) is the bias. what you're left with is the dark noise.

you could also point the camera at a flat white surface - that would give you a lot more noise (so much you probably don't need to worry about subtracting anything, if you just take the least significant bits from each pixel)

[i hope i'm right - i wrote astronomy image reducing software for a living]