Hello, Welcome to HCM. I can understand your concern regarding the skin condition. From your description it seems like you might be having some skin infection (Pityriasis versicolor or Tinea corporis) or it could be due to blood extravasation from the vein causing dark blue colored discoloration on the arm from which your blood have been withdraw but it is difficult to point specific diagnosis in absence of clinical examination. So,in my advice,you should see dermatologist or your pathologist where you had gone for blood tests who will examine it thoroughly to come at a certain conclusion. However,there is nothing to be worried much since this kind of lesion can be subsided by itself and without any specific treatment. Still,it would be better to see doctor at least once. Regards,