"Leaving part of one's wall unpainted Zecher Lachurban...Some Poskim rule it does not help to paint a 1×1 Ama space with black paint, and rather the space must remain unpainted...Other Poskim however justify those who do so and so is the custom of many to paint a black square on which they write Zecher Lechruban." (source)

"This week's article discusses the nature and laws of enactments that are Zecher LaChurban – in commemoration of the Destruction." (source)