Black Run (also known as Black's Run) is a tributary of Spruce Run in Union County, Pennsylvania, in the United States. It is approximately 4.7 miles (7.6 km) long and flows through West Buffalo Township and Buffalo Township. The watershed of the stream has an area of 4.72 square miles (12.2 km2). The stream has two unnamed tributaries and is not designated as an impaired waterbody. Its watershed is mostly forested, but there is some agricultural land.