What I've read in this area all comes back to "whether or not black coffee breaks the fast depends on the goal of your fasting", i.e., weight loss vs something more specific.

[Likewise for other things people don't always think about like vitamin / supplements.]

There is more info in [1][2] but tl;dr:

- Fasting for metabolic health/weight loss: likely does not break a fast

- Fasting for gut rest: does break a fast

- Fasting for longevity: likely does not break a fast

[1]: https://www.zerofasting.com/does-coffee-break-your-fast/

[2]: https://www.dietdoctor.com/can-use-cream-coffee-fasting