“Smurf” cartoon characters are blue, and a riddle is:
   
Q: What do you get when you cross a Smurf with a cow?
A: Blue cheese.
   
The joke was posted on Twitter on July 28, 2011 (when the movie The Smurfs was released), and has been printed on several images.
 
“A Smurf walks into a bar. The bartender says, ‘Why so blue?’” is another Smurf joke.
       
 
Wikipedia: Blue cheese
Blue cheese is a general classification of cheeses that have had cultures of the mold Penicillium added so that the final product is spotted or veined throughout with blue, or blue-grey mold and carries a distinct smell, either from that or various specially cultivated bacteria. Some blue cheeses are injected with spores before the curds form, and others have spores mixed in with the curds after they form. Blue cheeses are typically aged in a temperature-controlled environment such as a cave. Blue cheese can be eaten by itself or can be spread, crumbled or melted into or over foods.

The characteristic flavor of blue cheeses tends to be sharp and salty.
 
Wikipedia: The Smurfs
The Smurfs (French: Les Schtroumpfs; Dutch: De Smurfen) is a Belgian comic franchise centered on a fictional colony of small blue anthropomorphous creatures who live in mushroom-shaped houses in the forest. The Smurfs was first created and introduced as a series of comic characters by the Belgian comics artist Peyo (pen name of Pierre Culliford) in 1958, where they were known as Les Schtroumpfs. There are more than one hundred Smurf characters, and their names are based on adjectives that emphasize their characteristics, such as “Jokey Smurf”, who likes to play practical jokes on his fellow smurfs. “Smurfette” was the first female Smurf to be introduced in the series. The Smurfs wear Phrygian caps, which came to represent freedom during the modern era.
     
Twitter
James FW Thompson‏
@JamesFW
Head cheese = cow heads; blue cheese = smurf heads. I don’t even wanna know about swiss cheese. Oh, did you know that about blue cheese?
10:07 PM - 21 Jun 2009
     
IMDb (The Internet Movie Database)
The Smurfs (2011)
Quotes
Jokey: What do you get when you cross a Smurf with a cow? Blue cheese!
 
Twitter
DJ JD of Born To DJ‏
@iamdjjd
What do you get when you mix a #smurf with a cow? Blue Cheese (rim shot)
7:23 PM - 28 Jul 2011
 
Twitter
Jackson Scott‏
@JacksonScott_
RT @arianabarr: What do you get when you mix a smurf with a cow ? BLUE CHEESE! HAHAH
11:34 PM - 28 Jul 2011
 
Twitter
Jenny D ✨‏
@jennlove17
What do you get when you put a smurf and cow together?! Blue cheese !
10:01 AM - 29 Jul 2011
 
Twitter
♛New York’s Finest♛‏
@PoeticallyNEEK
Wat do you get when you cross a smurf with a cow….blue cheese!
10:16 AM - 29 Jul 2011
     
Cultjer (video from the 2011 movie The Smurfs)
Clumsy introduces Smurf Village
burnstingburnsting May 3, 2014 in THE SMURFS
0:34
What do you get when you cross a Smurf with a cow.
Blue cheese. Hahaha.
 
Twitter
Kian‏
@Kiancow
Joke of the day: what do you get when you cross a smurf and a cow?... Blue cheese! 🐮🌺🌊🍪 #cowcookies #JokeoftheDay #cowabunga #tamparep
11:17 AM - 2 Feb 2017