The Hoya 72mm 80B Hoya Multi-Coated (HMC) Glass filter is blue. It is 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.
Hoya makes the 80 series of filters in many sizes to accommodate the many uses this filter has. This filter should be used by the pro and amateur alike - weekend photo warrior or pro cinematographer and must be considered an essential addition to any photo arsenal.
Multi-coated to minimize reflection at the filter surfaces which reduces flare and ghosting.