Why is it more difficult to appreciate erythema in darker skin?

Erythema, or the amount of visible redness in the skin, is caused by increased blood flow and/or blood vessel engorgement in the dermis with the presence of oxyhemoglobin. If the epidermis is deeply pigmented, the red hues of oxyhemoglobin may be difficult to visualize. For this reason, the interpretation of patch testing for sensitivity to cutaneous allergens in black patients is challenging. This is an important point to remember, as many diseases that have erythema as a hallmark, such as seborrheic dermatitis or atopic dermatitis, may present more subtly in a black patient. Cyanosis is also difficult to perceive in a dark-skinned patient for similar reasons.

Ben-Gashir MA, Hay RJ: Reliance on erythema scores may mask severe atopic dermatitis in black children compared with their white counterparts, Br J Dermatol 147:920–925, 2002.