The 80 series of filters are blue. They are used to balance daylight film under artificial, tungsten or standard (household) incandescent lighting.
You've been given an assignment to do some slide copying on a copy stand. Unfortunately, you only have daylight film and you notice that the copy stand is loaded with 3400K photofloods. Not to worry! An 80B is a strong dark-blue filter that will balance your colors back to reasonable normality. This filter can also be used as a "conversion" filter for when you are shooting in a room with incandescent light (household light bulbs) and daylight film.
This filter allows you to "balance", i.e. bring back, correct color tone to your images. There is some loss of film speed that may require the use of a tripod but, at least, your photos will demonstrate the proper tonality they so richly deserve.