Why has Han's painting business been booming during the summer of 1942?

From the text:

For Hans Hubermann, this uneasy development was actually a slight reprieve. At an unfortunate time, good luck had somehow found its way into his painting business. 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. There were many houses in all parts of Molching in which he confiscated the window light from enemy eyes.