black, dark, evil, sinister
黑 centers on the color black and the qualities associated with it: darkness, lack of light, and by extension evil or sinister things.
黑 is a pictograph of a person with a tattooed or painted face, representing the color black or darkness. The modern form retains the general shape with the four dots at the bottom suggesting fire or soot.
The top part looks like a dark container, and the four dots at the bottom are like drops of black ink or soot. Together they make 黑, the color black.
For コク, imagine a black crow cawing 'koku, koku' as it flies through the dark sky: koku -> コク.