K is blacK. I believe it came from the German Karbon, in other words carbon or soot: black ink. The Germans were big in inks and dyes a hundred, hundred and fifty years ago when this was being worked out. Ever heard of IG Farben? Farben is paint. 
My grandpa was a cartoonist for the local paper and did a drawing for their Sunday Magazine (newsprint and not glossy). He would do the drawing and then have separate transparencies with each color painted in the areas it needed to go in. I guess then they would combine them and take a picture??? Or each transparency was for a different plate?
That’s interesting. I did not know that.
Would be for a different plate. Someone upthread mentioned the limited colour palette of newpaper comics? I suspect he simply picked the colours from a sheet, and worked out which colour areas would get which combinations and intensities of C, M, Y, and K.
As Sunspace says, each of the transparencies would be used to form a different plate, and then those plates would all ink the same piece of paper (I don’t know enough of the operation of presses to tell you exactly how that works). But that’s why you can have poor registration if the paper moves in between when the first and the last plate do their thing.
–Cliffy
All high-volume presses are roll-fed. In this case, there are a limited number of rollers that can hold images, usually no more than six. The different color images are wrapped around the rollers, and their ink is transferred to the paper as it passes beneath them. The images are usually in fairly close alignment due to the way the plates are held to the rollers, but generally there is a fine adjustment that will allow each color to be registered to all the others. As in all things mechanical, there are tolerances (including paper stretch) which prevent perfect alignment.
(a not-very-good description at wikipedia)
I believe most artists would use a red masking film called Rubylith that they would cut out and then affix to as many transparencies as there are colors.