墨

Bottom 土 - earth radical, the indexing radical, marking 墨 as an earth-product. Traditional Chinese ink was made by mixing pine soot with animal glue and moulding into sticks, dried like clay - so the soil radical fits the material rather than the colour. Pair with 砚 (inkstone), 笔 (brush), 纸 (paper) - the four treasures of the study.

Top 黑 - a sooty face above a fire, the very picture of blackness. Provides both the colour and a partial sound (hēi → mò is irregular but linked through Old Chinese). The dominant semantic clue: ink is the black substance. Same 黑 appears in 默 (silent - dark mouth), 黛 (eyebrow black).