The seating in a sports stadium called bleachers - - why are they called bleachers and were they first coined that for baseball?

According to this No they were not coined for baseball
https://en.wikipedia.org/wiki/Bleacher

A key feature of bleachers is that they are generally uncovered and unprotected from the sun; thus the wooden benches were "bleached by the sun." The term "bleachers" used in this sense can be traced back to at least 1889. The Dickson Baseball Dictionary states that the open seating area was called the "bleaching boards," as early as 1877. Dickson lists as a secondary definition the fans sitting in them. By the early 1900s, the term "bleachers" was being used for both the seating area and its occupants.
In modern usage the term "bleachers" almost always refers only to the seating area, and those sitting there may be called "bleacher fans," or "bleacherites." Terms such as Chicago's "bleacher bums," or Yankee Stadium's Bleacher Creatures are also used.

https://en.wikipedia.org/wiki/Bleacher