。
She went deathly pale at the sight.
Literal
She [topic-は] that scene [object-を] saw [て] pure-blue became.
まっ青 is the intensified form of 青 ('blue / pale'), with 真っ~ (or まっ~ in kana) acting as an intensifier prefix: 真っ赤 ('bright red'), 真っ白 ('pure white'), 真っ暗 ('pitch dark'), 真っ青 ('deep blue / pale as a sheet'). In Japanese, the color 青 covers both 'blue' and certain shades of 'pale' — including the bloodless face of someone shocked or frightened. ~になる ('become') captures the change of state. The te-form 見て links the seeing to the color change: 'saw and as a result paled.'