。
She had a pale face.
Literal
She [topic-は] pale face [object-を] was-having.
青ざめた is the past form of 青ざめる ('to turn pale') — literally 'blue-faded,' using 青 (blue/pale) + ざめる (to fade/blanch). Here 青ざめた modifies 顔 (face) as an attributive: 'a face that has turned pale.' 顔をしていた uses する in the sense of 'to have' or 'to wear' (a facial expression), marking this as an observed, ongoing past state.