What did Blanche "see" and "hear" that "disgusted" her?

Blanche saw her husband with another man.... an older man who'd been his friend (lover) for years.

It was because--on the dance-floor--unable to stop myself--I'd suddenly said--"I saw! I know! You disgust me..." And then the searchlight which had been turned on the world was turned off again and never for one moment since has there been any light that's stronger than this--kitchen-candle...