1 Make a sudden flinching movement out of fear or pain.
2 to make or become pale or white; variant of blanch
His cheek did not blench, his lips quiver, nor his limbs tremble. (R Shelton Mackenzie, Bits of Blarney)
We blench and retreat just like soldiers who are forced to abandon their camp because of a dust-cloud raised by stampeding cattle, or are thrown into a panic by the spreading of some unauthenticated rumour. (Seneca, Letters to Lucilius,)
Old English blencan 'deceive', of Germanic origin; later influenced by blink. blench (sense 2) is a variant of blanch, although in practice the two senses are very difficult to disambiguate. (Oxford English Dictionary)
Before 1000; Middle English blenchen, Old English blencan; cognate with Old Norse blekkja, Middle High German blenken (Dictionary.com)