“What is black when it is clean and white when it is dirty?”/“A blackboard.”

A popular school riddle is:
 
Q: What is black when it is clean and white when it is dirty?
A: A blackboard.
 
The riddle has been cited in print since at least 1939.
 
   
8 July 1939, Wilmington (DE) Morning News, “Riddles,” pg. 12, col. 7:
What is black when it is clean, and white when it is dirty?
== Margarette Cashman.
A blackboard.
 
26 April 1951, Marengo (IL) Republican-News, “District No. 8 School News,” pg. 13, col. 2:
What is black when it is clean and white when it is dirty?—the blackboard.
Dolores Aten
 
26 July 1952, Journal and Guide (Norfolk, VA), “Riddles,” pg. 16, col. 4:
What is black when it is clean and white when it is dirty?
Blackboard.
 
3 January 1958, Austin (TX) Statesman, “Fun Time—The Riddle Box” by A. Leokum, pg. 10, col. 5:
What is black when it’s clean and white when it dirty?
A blackboard.
 
Google Books
101 American English Riddles:
Understanding Language and Culture Through Humor
By Harry Collis
Lincolnwood, IL: Passport Books
1996
Pg. 23:
What is black when it is clean and white when it is dirty?
A blackboard.
 
Google Books
Managing Text Messaging
By Shuchita Ghai
Elgin, IL: New Dawn Press
2005
Pg. 94:
Q: What is black when it is clean an Sc white when it is dirty.
A: A blackboard.
 
Google Books
Penguin Pocket Jokes
By David Pickering
London: Penguin Books UK
2006
Pg. ?:
What’s black when it’s clean and white when it’s dirty?
A blackboard.
 
Google Books
More One Liners, Jokes and Gags
Compiled by Grant Tucker
London: Biteback Publishing
2013
Pg. ?:
What is black when clean and white when dirty?
A blackboard.