See here. Apparently it’s because of electrons (how’s that for a cogent explanation?). I’ve watched a lot of the Leonard Bernstein/New York Philharmonic Young People’s Concerts and the early B&W ones had a lot of “blooming” as it’s called happening in the brass section.
Fun fact: Men’s blue dress shirts are a thing because of camera bloom. Back in the early days of broadcast TV, men just wore white dress shirts, but they were too bright for the camera, so the TV people started wearing blue shirts so as to appear white on the camera, but not be as bright. Eventually it just caught on as a style.
