When I see that single blue drop in the middle of the pad, I'm thinking, "That's exactly how my period works."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 show up at all. 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? See also Water Is Blue.
— Maxi Pad commercial lampshading this trope.