The blue milk that Luke skywalker drinks as seen in Star Wars A New Hope.
Red - Skimmed or Low Fat
Green - Semi Skimmed or Regular
Blue - Full Fat
As most people find the Full Fat version the most enjoyable, the phrase Blue Milk has become synonymous with things that are better than average, or have the ability to make you happier than average.
The term "Blue Milk" does not often refer to anything specifically, but suggests improving something in the same way that Full Fat milk is an improvement over Low Fat.
Person 1: "I feel very sad today :("
Person 2: "I think you need some Blue Milk!"
Person 1: "This computer is really old, I can barely even get online any more"
Person 2: "You should put some Blue Milk on it."