To blanch or not to blanch, that is the question.
Blood extravasation into dermal layers –> Purpura –> Non-blanching
Purpura describes the red-purple lesions caused by extravasation of blood into the skin or mucous membranes. These can be small (petechiae <3mm) or larger (ecchymoses). These are caused by infection (meningococcal septicaemia), coagulopathies, and the vasculitidies (the purpura will be palpable).
As the blood products are in the epidermal layers, applied pressure (i.e. the glass test) does not reduce the red discolouration of the skin.
Inflammatory vasodilation of blood vessels –> Erythema –> Blanching
Conversely, erythematous rashes are caused by localised vasodilation and thus, blood products are retained in the vasculature. When pressure is applied, blood in these vessels is squeezed away from the site and so the skin blanches.