Diascopy is a test for blanchability

Blanch (medical)

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....

 performed by applying pressure with a finger or glass slide and observing color changes.

It is used to determine whether a lesion is vascular (inflammatory) or nonvascular (nevus) or hemorrhagic (petechia or purpura). Hemorrhagic lesions and nonvascular lesions do not blanch; inflammatory lesions do. Diascopy is sometimes used to identify sarcoid skin lesions, which, when tested, turn an apple jelly color.