What does Hans add to the paint to make it darker and blacker?

Papa added coal dust to make the paint blacker.

People with blinds were desperate enough to enlist his services to paint them. His problem was that black paint was normally used more as a mixer, to darken other colors, and it was soon depleted and hard to find. What he did have was the knack of being a good tradesman, and a good tradesman has many tricks. He took coal dust and stirred it through, and he worked cheap.