Is there a word play? Does it just mean shut up

There are none. This is just the tradition in this movie. And yeah, it just means shut up.

For information, some people now shout loudly “TA GUEULE !” when someone use the sentence “Ca va être tout noir”. ^^

Dans ce contexte “ta gueule” peut tout simplement se traduire par “shut up”.
Il n’y a pas de jeu de mots, mais “ça va être tout noir” est devenu un mème en français.

Voici l’extrait en question :
Ca va être tout noir! TA GUEULE! rrr (YouTube)

Yes “Ta gueule” is the short way for “Ferme ta gueule” and it’s really rude and irrespectful, specially when said loudly. I don’t know the real translation for it. “shut up” just means “tais-toi” … a little bit less irrespectful!
But “ta gueule” when said gently with a smile can be quite sweet in fact.