I have only very little expericence with AZO. But what you are saying makes sense to me.

Since AZO is only sensitive to blue light, your blue channel reading may give direct results. The problem is, that you must use the blue channel to measure yellow/brown stain even if your paper is sensitve to other colors as well. Your lucky case is that the blue channel actucally "sees" like the AZO paper does. But this is not true for most papers, including graded.

Since the stain is actually a color density (that's why you are using a color densitometer to determine it), one usually cannot ignore the color issue. One must look at it with the paper's eyes.