Matt Black vs Slate Grey

How close are they?

ΔE 26.2, loose: different enough that you should pick on purpose. Slate Grey is noticeably lighter, more saturated than Matt Black.

Can I use Slate Grey instead of Matt Black?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Slate Grey is noticeably lighter, more saturated than Matt Black.

Can I use Matt Black instead of Slate Grey?

The same difference applies the other way round: Matt Black is noticeably darker, more muted.

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.