Very high values of any atmosphere setting are going to produce strange results. The defaults are "Earth-like normal". I don't know what specific effect Bluesky Density has at higher values, but it does seem to have some qualities of "Haze" to it along with adding color. Note also that Bluesky Density and Redsky Decay are inextricably linked, so what you may be seeing is the correspondingly very high value of Redsky Decay which seems logical to cause a darkening (because if increased light decay).
- Oshyan