ΔE 8.5, close: similar, with a visible difference side by side. Blue FS15050 is a little lighter, more muted than Dark Sky.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Blue FS15050 is a little lighter, more muted than Dark Sky.
The same difference applies the other way round: Dark Sky is a little darker, more saturated.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Dark Sky on the colour wheel See Blue FS15050 on the colour wheel
All matches: Dark Sky equivalents · Blue FS15050 equivalents