Dark Sky vs Blue Black

How close are they?

ΔE 15.9, loose: different enough that you should pick on purpose. Blue Black is much more muted, cooler than Dark Sky.

Can I use Blue Black instead of Dark Sky?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Blue Black is much more muted, cooler than Dark Sky.

Can I use Dark Sky instead of Blue Black?

The same difference applies the other way round: Dark Sky is much more saturated, warmer.

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.