A wide variety of substances may cause hives in sensitive people, including foods,
drugs, and insect bites or stings. Common culprits include:nuts, especially peanuts, walnuts, and Brazil nuts,fish, mollusks and shellfish ,eggs ,,wheat and milk .
You can treat it with antihistamines, such as diphenhydramine (Benadryl).Also cotinue to apply calamine lotion.
If you can figure out what set this off and avoid it , the rash will be gone.The black marks will be a result of scratch marks.