Salutation; refers to the artificial horizon indicator on an aircraft instrument panel. Blue represents sky and black represents the ground. If the black side is up, this will not end well. Similar to truckers' salutation of "Keep it between the paint" and satirical theatrical salutation of "Break a leg."
Said to a spouce: "See ya later, honey; keep the blue side up!"
To a friend: "Remember to keep the blue side up!"
What to say to your fellow gang mates when smoking a fat blunt behind ESK.
You must always say this when you approach a fellow gang man, this will assert your dominance to the other gang man and make I'm look like a little wetty cuz
Gang Man1: Ayoo wagwan blud cuz I heard you been sayin shit fam
Gang Man2: Nah fam I wasn't saying shit ya get me fa-
Gang Man1: SHUT UP BLUD
Gang Man1: Ah safe fam
A sticker in messenger used to indicate, "I have stopped talking. You should also stop talking."
Although it also can imply that you like something, so it's still polite.
Friend: Are we still on for *that stupid event that you haphazardly agreed upon* tomorrow evening?
You: *Blue thumbs up*
Friend: Great! See you there!