[[quoteright:350:http://static.tvtropes.org/pmwiki/pub/images/kotex_blue_liquid_5940.jpg]]
[[caption-width-right:350:[[http://www.youtube.com/watch?v=FRf35wCmzWw The ads on TV are really helpful, because they use that blue liquid. And I'm like, "Oh! That's what's supposed to happen."]]]]
->''When I see that single blue drop in the middle of the pad, I'm thinking, "That's exactly how my period works."''
-->-- Maxi Pad commercial lampshading this trope.

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 ''[[BodyHorror gone horribly wrong]]''. Clear [[RuleOfPerception wouldn't show up at all]]. Meanwhile, the only fluid you're going to associate with blue is good old pure, healthy [[WaterIsBlue water]].

Humourously, a blue liquid test will nearly always have a small disclaimer on-screen, reminding us that this is a "dramatization". [[ViewersAreMorons 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 "YesButWhatDoesItDo?" 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?

See also WaterIsBlue.
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!!Examples:
* 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.
* Pampers diapers. During a literal SideBySideDemonstration they poured a puddle on TheLeadingBrand 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.
* Even true, often, for things like Bounty paper towels. (Though more recently they use water or juice for a realism effect).
* Averted in a new pad brand here in Brazil. But it didn't change that much, they use a green liquid.
* Pretty much any ad for menstrual products. (This does include the Always pads.)
** This is partly because even ''advertizing'' menstrual products was banned until the 1980s, and then it was only allowed if any blood shown was not red or a realistic colour.
* {{Averted}} in a (relatively) old ad for a brand of 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 on the floor and uses the pad being advertised to soak it up before the detectives could see it. Needless to say, the ad was quickly pulled.
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