Inspect Black (#000000) in HEX, RGB, HSL, LCH and OKLCH, check its contrast, and copy the value you need.
Black
#000000
White text reads best on this background, at 21:1.
Black is one of the 148 CSS named colors, and resolves to #000000 in every browser. In numbers that is rgb(0, 0, 0) and hsl(0, 0%, 0%). Perceptually it is a very dark, near-neutral color: OKLCH puts its lightness at 0% and its chroma at 0. It carries almost no chroma, so it behaves as a neutral rather than as a hue, and belongs with the grays.
White text clears WCAG AA on a Black background at 21:1, and AAA as well. Its darkness makes it a natural surface for light text, or a strong anchor at the bottom of a shade scale.
The closest CSS named colors to Black, measured with CIEDE2000, are DarkSlateGray (#2F4F4F), DarkSlateGrey (#2F4F4F), MidnightBlue (#191970). If Black is not quite right, those are the nearest keywords to try before reaching for a custom value.
HEX
#000000
RGB
rgb(0, 0, 0)
HSL
hsl(0, 0%, 0%)
HWB
hwb(0 0% 100%)
CMYK
cmyk(0%, 0%, 0%, 100%)
LAB
lab(0 0 0)
LCH
lch(0 0 0)
OKLCH
oklch(0% 0 0)
WCAG asks for 4.5:1 between body text and its background. On #000000 white text measures 21:1 and black text 1:1, so white clears it and black does not.
White text on Black
WCAG 2.2
21:1
APCA
Lc 107
Passes AA body text
Passes AAA body text
Passes AA large text
Black text on Black
WCAG 2.2
1:1
APCA
Lc 0
Fails AA body text
Fails AAA body text
Fails AA large text
Check Black against any other color in the contrast checker.
Black sits at 0% OKLCH lightness. Holding its hue and chroma and moving only that value carries it up to #F8F8F8, which keeps the family recognizable instead of letting it drift toward gray the way a plain HSL ramp does.
#151515
20% lighter
#454545
40% lighter
#7C7C7C
60% lighter
#B8B8B8
80% lighter
#F8F8F8
100% lighter
Build a full accessible scale from #000000 in the shade generator.
Black barely moves under any of the four common forms of color blindness, the largest shift being 0 CIEDE2000 units under protanopia. It stays recognizable, though it still should not be the only thing carrying meaning.
Protanopia
#000000
Deuteranopia
#000000
Tritanopia
#000000
Achromatopsia
#000000
The nearest CSS keywords to #000000, ranked by perceptual distance rather than by how close their hex digits happen to look.