In commercials featuring absorbency tests, particularly menstrual products and diapers, 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 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, or might squick out viewers for other reasons. Meanwhile, the only fluid you're going to associate with blue is good old pure, healthy water.
Humorously, 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 somewhere actually has blue periods.
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 and diapers uses this trope, all around the world. In the case of menstrual products, advertising them at all was banned until the 1980s, and then it was only allowed if any blood shown was not red or other realistic color. Compare No Periods, Period.
See also Water Is Blue. Nothing to do with Blue Blood; either the metaphorical aristocratic kind, the TV show named after a slightly-different metaphorical kind, or Alien Blood that's literally blue. Compare Rainbow Puke for a similar kind of color substitution to avoid squicking out the audience.
- 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.
- Square Root of Minus Garfield: Lampshaded in "Blue Monday". Garfield chases a fly and jumps into a toilet, with a blue "SPLOOSH" panel before it. In the original comic, the panel is yellow. The author notes that Garfield landing in an unflushed toilet is more disgusting than the ambiguous original comic, which would have been black-and-white when it was first printed, and colored it blue "in accordance with TV adverts where any bodily fluid is always represented by blue liquid".
- This trope has inspired no shortage of snark, even from the companies themselves.
- In the Family Guy episode "Tea Peter", when Peter is talking about the bad things that are happening without the government, one of things he brings up is that tampon commercials no longer use blue liquid in their demonstrations. We are then shown a Cutaway Gag of the Griffins reacting in disgust to a commercial.
Stewie: Make it blue, thats always been the deal! You show whatever you want but make it blue!
- Xavier: Renegade Angel has a fake commercial in one episode for special cookies with blue chocolate chips. As the commercial goes on, its revealed that there was a mix-up with the cookie and tampon factories and they are now selling tampon cookies with blue chocolate chips in them.
- This trope is also used by toilet capsules that prevent the bowl from staining (ie, the things that turn the water blue). Colorless capsules exist, but they don't tell you when it's time for a new one, and any other color besides blue would not be what you'd want to see when you open the lid.