That being said, you can argue, and people do, that Herbert blows the description of the sky himself, as it's described two ways, which most people ignore or attribute to one of them being at altitude and one being at....sea (of dust) level.

at the minimum, however, the sky should be *not* blue.
Although I suppose blue at sunset and sunrise, like on Mars, but it still would not be OUR blue, which requires at least three things Arrakis lacks.

(a yellow sun, a dense atmosphere, and moisture in the atmosphere, or put more nerdly, Rayleigh diffusion, which requires particles so small that if they were the dust on Arrakis not moisture everyone would have silicosis and Mie scattering which wouldn't happen with cubic dust)