Is #000000 on #679db3 accessible?

Yes for WCAG 2.2 level AA: the contrast ratio is 7.05:1, above the 4.5:1 needed for normal text. It also meets AAA (7:1).

What is the contrast ratio of #000000 and #679db3?

7.05:1, calculated with the WCAG 2.2 relative luminance formula. The ratio is truncated, not rounded, so it never shows a failing pair as passing.

7.05:1

Black text (#000000) on a blue background (#679db3) meets WCAG 2.2 level AAA for normal text.

Large text sample

Normal body text in #000000 on #679db3. The quick brown fox jumps over the lazy dog.

This pair already meets AAA. Nothing to fix.

Simulated with the Machado et al. (2009) model at full severity.

Hex, rgb() or hsl(). Alpha is composited.

Text colour

Background colour

Large heading text

Normal body text at 14 px. The quick brown fox jumps over the lazy dog.

Outlined buttonA text link

Simulated with Machado et al. (2009), full severity.

7.05:1

WCAG 2.2 contrast verdicts
Check	Criterion	Needs	Result
Normal text	1.4.3 AA	4.5:1	Pass
Large text	1.4.3 AA	3:1	Pass
UI components & graphics	1.4.11 AA	3:1	Pass
Normal text	1.4.6 AAA	7:1	Pass
Large text	1.4.6 AAA	4.5:1	Pass

Same hue, lightness adjusted in OKLCH.

AA — 4.5:1

Already passes.

AAA — 7:1

Already passes.

Large text means at least 24 CSS px, or 18.66 CSS px bold. Ratios are truncated, never rounded, so 4.49:1 is shown as failing 4.5:1.

Is #000000 on #679db3 accessible?: Yes for WCAG 2.2 level AA: the contrast ratio is 7.05:1, above the 4.5:1 needed for normal text. It also meets AAA (7:1).
What is the contrast ratio of #000000 and #679db3?: 7.05:1, calculated with the WCAG 2.2 relative luminance formula. The ratio is truncated, not rounded, so it never shows a failing pair as passing.