Dark Reaper vs Blue Mountain

How close are they?

ΔE 15.6, loose: different enough that you should pick on purpose. Blue Mountain is a little lighter, warmer than Dark Reaper.

Can I use Blue Mountain instead of Dark Reaper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Blue Mountain is a little lighter, warmer than Dark Reaper.

Can I use Dark Reaper instead of Blue Mountain?

The same difference applies the other way round: Dark Reaper is a little darker, cooler.

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.