Question - What is so-called “black spot poison ivy”?
Answer - The cited reference point out black spot poison ivy is an atypical appearance [of the poison ivy rash that] occurs due to the oxidation of urushiol upon exposure to air in a warm, humid envi- ronment. High-enough concentrations of urushiol on the skin are rarely achieved and maintained for this reaction to occur but, when present, cause both an irritant and allergic contact dermatitis. Note that it is this reaction that forms the basis of the toxicodendron field test—blackening of the resin when sap is directly applied to white paper. (Pittman MA and Lane DR. Black spot poison ivy: under the over of darkness. 2013 J Emerg Med 44(4): e331-e332)