From BR Bullpen

Bleachers is a term used to describe the raised, tiered stands found by baseball fields. Bleachers are long rows of benches, often consisting of alternating steps and seats. They range in size from small, modular, aluminum stands that can be moved around fields to large permanent structures that wrap around a field. Bleachers can be hollow underneath, aside from their support structures. Some bleachers have locker rooms underneath them.

A key feature of bleachers is that they are typically uncovered, i.e. unprotected from the sun; thus the seats, and the fans themselves, are subject to "bleaching" from prolonged exposure to the sun. Some sources claim that the term is primarily derived from that feeling of being bleached by the sun while sitting in them. However, The Dickson Baseball Dictionary discusses the term in greater depth. The open seating area was originally called the "bleaching boards", as early as 1877. By the early 1900s, the term "bleachers" was being used for both the seating area and its inhabitants. Thus, Dickson lists the fans themselves as "bleachers", as a secondary definition. Other terms, such as "bleacher seats" and "bleacher entrance" and "a home run into the bleachers", are ambiguous enough that they could refer to either the seats or the fans. However, in modern usage the term "bleachers" almost always refers to just the seating area, and its participants may be called "bleacher fans", or "bleacherites", or (in Chicago) "bleacher bums".

In major league stadiums, the bleachers are usually located beyond the outfield fences. However, center-field bleachers are located in the line of sight of the batter, and the presence of fans makes it difficult for the batter to pick out the ball. As a result, most stadiums have vacant areas or black backgrounds where the seats would be. This is known as the batter's eye. Yankee Stadium has featured black-painted vacant bleachers -- nicknamed the black by baseball fans -- since it reopened in 1976 after a two-year renovation. In the original Stadium, the center-field section of the bleachers was originally occupied, though from the 1950s they were obscured with a portable screen. Bleachers can be used for all sports known.