。
She turned pale at the sight.
Literal
She [topic-は] that scene [target-に] became-blue.
青くなる ('turn pale,' lit. 'become blue') captures the bloodless complexion of someone shocked, ill, or frightened. The に here marks the trigger of the reaction (the scene), the same に that appears in ~に驚く ('be surprised at'). 青い in Japanese covers both 'blue' and certain shades of 'pale' — the same color word handles both, even though English splits them into separate concepts. This color overlap is a recurring oddity for English-speaking learners: 青信号 ('green traffic light') is also literally 'blue light.'