1.
dark ink; thick, deep-black sumi ink
Sumi ink ground to a deep, dark black, as opposed to 薄墨 (pale, watered-down ink). Used in calligraphy and traditional writing, and standard for congratulatory or formal correspondence.
The counterpart of 薄墨 (pale ink). In traditional correspondence the depth of the ink carries meaning: deep black ink is used for happy or formal occasions, while pale ink signals mourning.