i-adjective
deep black; jet-black
Describes a very dark, intense black color. The prefix か- adds emphasis, similar to 'deep' or 'jet' in English.
彼女はか黒い髪をしている。
She has deep black hair.
か黒い雲が空を覆った。
Jet-black clouds covered the sky.
黒い is the standard word for 'black'. か黒い adds the emphatic prefix か-, making the blackness more intense and vivid.
The prefix か- is an old emphatic element, also seen in words like か細い (かぼそい, 'very thin'). The exact origin is uncertain, but it intensifies the following adjective.