When skin is blanched, it takes on a whitish appearance as blood flow to the region is prevented. This occurs during and is the basis of the physiologic test known as diascopy

Diascopy

Diascopy is a test for blanchability performed by applying pressure with a finger or glass slide and observing color changes.It is used to determine whether a lesion is vascular or nonvascular or hemorrhagic . Hemorrhagic lesions and nonvascular lesions do not blanch; inflammatory lesions do...

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Blanching of the fingers is also one of the most clinically evident signs of Raynaud's phenomenon

Raynaud's phenomenon

In medicine, Raynaud's phenomenon is a vasospastic disorder causing discoloration of the fingers, toes, and occasionally other areas. This condition can also cause nails to become brittle with longitudinal ridges. Named for French physician Maurice Raynaud , the phenomenon is believed to be the...

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Blanching is prevented in gangrene as the red blood corpuscles are extravasated and impart red color to the gangrenous part.