I think it's from laundry detergent. People will pay more for a white powder with useless colored crystals in it than they would for the plain white powder. Psychologically, the implication is that the crystals are some sort of secret sauce that makes the powder work better, or is responsible for all the brand-linked advertising claims.

Since the actual chemistry is over most people's heads, the "blue crystals" are where they invest all the magic that makes it work just like the ads claim it will.

At the time, a lot of people were regularly using laundry bluing as a whitening agent along with soap (not another detergent agent, real, honest-to-goodness soap, like Sunlight Soap or Ivory Snow); it was difficult to tell those people that a more effective detergent, by itself, could accomplish what soap and bluing together had been doing. Thus blue crystals in detergent.