So if I got you correctly: You want the grain to be as light as possible. Then stain the guitar blue, without it affecting the grain filler too much?

Maybe I am just going to say crazy stuff now, and I have never tried this and I would try it on an off-cut first. But what if you could make the white grain filler saturated with a neutral/repelling substance like acrylic clear-coat, -mix, -apply, -sand back, -stain? Maybe this would abstain the filler from taking on too much of the stain.

Good luck!