Tightly tied to the
户口 (hùkǒu, household registration) system. Children born outside the one-child policy quota (now repealed), illegal migrants, or those whose parents never registered them all become
黑户 — denied schooling, healthcare, ID cards, and travel. The black market uses the same word for any unlicensed business.
黑 here means 'illegal / off-the-books', not 'black' literally.
