The walls of the old house were stained pitchy from years of soot.

pitchy 是由名词 pitch（沥青、焦油）加上形容词后缀 -y 构成的形容词。pitch 本身指黑色粘稠的物质，pitchy 形容像沥青一样漆黑的状态，常用来描述被烟熏或污染后变黑的表面。