Colloquial spelling of "Balloon", sometimes used metaphorically as an insult referring to someone who has known negative attributes. Why call someone a balloon? No real answer.
"Here, look at that drunkfella over there" "Aye, seen him earlier, hes a bloon."
Bloon is a balloon from the game Bloons Tower Defense developed by Ninja Kiwi. it should be capitalized because Bloon is made-up by Ninja Kiwi, and is also in the title.