In commercials featuring absorbency tests, the liquid being absorbed will be bright blue. This is a deliberate attempt to avoid unsavory resemblance to any kind of bodily fluids, even though that is generally the product's intended use. Think about it — red/pink/purple, yellow/orange, or brown? Pretty obvious. Green or black would most likely put you in mind of the same kinds of fluids, except with something gone horribly wrong. Clear wouldn't easily show up on comparison shots, and having a clear, but translucent liquid would squick out older viewers for...other reasons, shall we say.. Meanwhile, the only fluid you're going to associate with blue is good old pure, healthy water.
Humourously, a blue liquid test will nearly always have a small disclaimer on-screen, reminding us that this is a "dramatization". Just so the audience didn't think that somebody was actually peeing blue liquid onto the whatever.
Blue liquid was first used in place of the others probably for the simple desire to not Squick out viewers, especially those who might be eating at the time. This, before the Internet age, placed some of these products in the "Yes But What Does It DO?" class for viewers under a certain age. Seriously, how does one tell a Poise pad from an Always pad if you don't know the liquid color?
Pretty much any ad for menstrual products uses this trope, all around the world. This is partly because even advertising menstrual products was banned until the 1980s, and then it was only allowed if any blood shown was not red or other realistic colour. Compare No Periods, Period.
See also Water Is Blue.
- Ads for Colgate Toothpaste used to use the blue liquid to demonstrate how fluoride gets into teeth, comparing it to the blue soaking into a stick of chalk(with the obligatory disclaimer not this fast).
- Pampers diapers. During a literal Side-by-Side Demonstration they poured a puddle on The Leading Brand and then moved over to the Pampers while still dripping the liquid so it was just a line of liquid. No wonder the other one was wetter.
- Ads for Bounty paper towels used the blue liquid. (Later they started to use water or juice for a realism effect.)
- Moony diapers, from Unicharm Corporation, uses green slime to represent loose stools.
- Averted in a 1990s ad for Libra pads. The ad involved a murder in a gallery (or somewhere), then when the detectives and curator (who did the murder) arrived at the scene, the curator sees blood (or water) on the floor and uses the pad being advertised to soak it up before the detectives could see it. The ad was quickly pulled.
- Mentioned in "Only the Good..." of Red Dwarf. Robot Kryten finds out about women menstruating for the first time in his lifenote . Dave Lister pranks him, so Kryten behaves inappropriately to Kristine Kochanski. Kryten assumes she wants to pour "all that blue stuff" over things.
- This trope has inspired no shortage of snark, even from the companies themselves.