The K in CMYK stands for “Key”, but the answer is much more interesting than that. The “key plate” is said to add the “detail” to a printed image. This is true in that the black plate in a four color process print pushes the contrast and creates “detail”. Many people suggest that the theory of using K instead of B because it may be easily confused with “Blue” is a myth. While it is highly speculative what the reasoning is, there are context clues as to why it may actually be true.
Mark Gatter, in the book he published titled “Getting it right in print”, states the “key plate” is used align the other plates, but does not mention why it is called “key”. While the term today may have adopted the meaning for the process