I used to call black people くろじん, either because I mistakenly read 黒人{こくじん} as such or because I heard it somewhere (or both), until I was told by another Japanese learner that こくじん is more politically correct.
Looking into it, I can't find the word in the dictionary.
I actually used くろじん for a while and was never corrected on it. I learned こくじん by asking what it meant, after someone said it.
Is this all my mistake, or is it actually a word? If so, is it derogatory?
It is not a word. Perhaps people understood what you were saying and had not bothered to correct you. When on-reading is mistakenly read as kun-reading, it is often easy to tell what you mean even if it is not correct.
Oddly enough I was referred to as くろじゃ by elementary school students and one old man when I went to Japan for a ten day exchange. I figured out こくじん was the correct term before those incidents, and I became confused after hearing natives say another way.
