What's commonly described as "black eyes" is, on close and well-lit inspection, almost always a very dense dark brown — true black, with essentially no light reflectance, does not occur naturally in the human iris.
"Black" here is a common descriptive phrase, not a formal classification category in eye-colour genetics or ophthalmology references — this comparison exists to explain that distinction, not to compare two biologically different categories.
This is one of the categories most affected by camera sensors and phone processing, which can crush shadow detail and remove visible brown warmth entirely.
No photograph or online colour-picker tool can definitively classify which of these categories a specific pair of eyes belongs to — classification is inherently somewhat subjective, and lighting/camera effects change the apparent result.