to be dampened (of the mood, interest, etc.); to be spoiled (e.g. of the atmosphere); to become subdued; to turn sour; to become unamused; to become bored; to become apathetic
to turn white; to fade (to white); to grow light (at dawn)
場の雰囲気が白けた。
The atmosphere turned flat.
その発言のせいで場が白けている。
That comment put a damper on things.
雰囲気が白けないよう必死に話題を変えた。
I desperately changed the subject so the mood wouldn't sour.