tatooing
黥 centers on tattooing, specifically the practice of marking the skin with ink, often as punishment or identification.
黥 combines 黑 (black, ink) with 京, which likely contributes the sound. The character refers to tattooing, especially punitive marking with ink.
Black ink (黑) is applied to the skin, and the right side 京 looks like a stylized needle or tool pressing into flesh. Together they create a tattoo.
For ケイ, imagine a tattoo artist named Kay inking a design: Kay -> ケイ, and the black ink marks the skin permanently.