。
She managed to escape under cover of darkness.
Literal
She [topic-は] darkness [in/into-に] disappearing-into [and-て] managed-to-escape.
闇にまぎれる ('blend into / disappear into the darkness') is a fixed atmospheric phrase common in spy stories, samurai tales, and noir fiction; まぎれる ('disappear into, get mixed up with') describes losing one's outline against a busy or dim background. 逃げとおせた is the potential past of the compound verb 逃げ通す: 逃げる ('escape') + ~通す (auxiliary 'do all the way through'). The auxiliary 通す here promises completion — not just running but escaping all the way through to safety.