this is what i remember (but its not from a text book)...when you push on an erythematous lesion on someone's skin and it blanches, its called DIASCOPY (a dermatologist told us that, so i'm not sure if most people are familiar with the word). Lesions that are due to vasodilation (like an allergic reaction or hives or something like that) should blanch because its just erythema from the area being flushed.....if its actually blood under the skin (purpura, ecchymoses) then pushing on the reddened area of skin won't make it blanch because theres actually blood underneath
hope this helps
