黒字
くろじ · kuroji · JLPT N1 · noun
black
💰 kuro (black) + ji (figure): books show surplus in black ink, loss in red
The opposite is 赤字 (akaji, red figure), since deficits were written in red ink. Black figures mean income exceeds spending, so the account is in the black.
字 — roof 宀 + child 子
Originally depicted a child 子 under a roof 宀, suggesting birth or rearing a child in the home. The meaning later extended to 'to give birth, nurture' and then to 'written character', since characters were seen as offspring derived from earlier pictographs.