Most sources list it as 0 or 1 calories. Additionally considering that caffeine may increase your RMR ("Resting Metabolic Rate") by 3% or more[0][1], if you want to be "precise" it may be more accurate to say that black coffee contains negative calories.

So how precise do you need it to be? 0, 1, <5, or -1, -5 calories? Or is this just pedantic nitpicking?

[0] https://www.ncbi.nlm.nih.gov/pubmed/7486839

[1] https://www.ncbi.nlm.nih.gov/pubmed/2912010

5 cals can really be rounded down to zero. It's important to also focus on the big picture.

This makes me think of premature optimization. There is such thing as energy homeostasis in biology. Most things about diet are going to be imprecise, and the target system is adapted to imprecision.