blind side

1. The side on which one's vision, especially the peripheral vision, is limited or obstructed.
2. The side away from which one is directing one's attention.

or blind·side (blīndˈsīdˌ)

1. To hit or attack on or from the blind side.
2. To catch or take unawares, especially with harmful or detrimental results: “The recent recession, with its wave of corporate cost-cutting, blind-sided many lawyers” (Aric Press).