[roger-wilco-66]

I agree it's too dark, I also thought that a couple of times. The picture below was made at around 6000meters / ca. 20k feet.
I think it depends also on weather / humidity conditions (haze) of the air - the clearer the darker, but in the sim its generally way too dark at that level.
Now at 35k feet this is different, I think, I've seen some pretty dark blue skies on my flights up there. I also have pictures of that.

Cheers,
Mark