The darkening is likely related to the lower overall reflectivity of the paper due to the matte finish. Matte finishes by design reflect environmental light from all angles in a diffuse manner rather than directly like a mirror with gloss papers.

In practical terms, that means the blackest blacks of a matte print are lighter than with a gloss paper because environmental light from all angles hit the matte and reflect it to the eye equally from the entire surface area. To compensate when printing you have to lower the overall contrast of the image to raise the blackest blacks enough above the paper threshold reflectivity to still yield image detail in the areas just above black.

If you make two identical prints adjusted perfectly for the glossy finish to matte and glossy paper you will get fewer intermediate tones with the matte finish. Areas could be darker or lighter depending on how a particular paper absorbs the ink from the jet.

It's actually a phenomenon throughout the print but will be more noticable with continuous area like the sky.

Forgot to mention - Hijack is great. Thanks.