。
She blushed.
Literal
She [topic-は] red became.
赤くなる literally means 'to become red' but idiomatically means 'to blush.' This is a natural i-adjective + なる construction: 赤い → 赤く + なる = 'become red.' Japanese describes blushing through color change rather than the English focus on the facial sensation. The same pattern works for other color-based expressions: 青くなる ('turn pale/blue'), 白くなる ('turn white').