A public toilet, so-named because of the color of its water, which has a blue hue due to the use of a disinfectant product which is released with every flush. (Also, coincidentally -- and appropriately, the title of a Brooke Shields movie.)
Gas Station Manager:
"Goddamit, Zeke. That last fat bastard who pulled in here left a big ol' stinkin' turd floatin' in the blue lagoon. Son-of-a-bitch won't flush. Now get in there and fish it out."
The ability of any two people to fall in love when there is no one else available.
Also the ability of the only person in a group of a certain gender to become the object of desire for the rest.
See also: proximity infatuation
A: "So how did you two get married?"
B: "Completely Blue Lagoon Syndrome. We were stranded in an elevator together over Christmas Break, so it was inevitable."
A: "I don't get it, everybody at work thinks Suzi is such a babe!"
B: "Dude, Suzi is the *only* female in your office, of course they think she's hot, it's Blue Lagoon Syndrome!"