Yeah, Ana, neither could I. Went all over the game environment, thinking there might be an HOG that I missed that would give me the blue ball. Nada. I went looking for a WT and saw the same one you linked to. No blue ball mentioned.
I click on this puzzle and she says, "A blue ball is missing." And that is that. I'm sure it is just me. I don't usually play HOGs. Hoping someone will have an answer. It is a fun game, 'cause it isn't just HOGs, but puzzles, too.