There isn't a blood test for black widow bites. The best way to diagnose a black widow bite is to feel the sting and look down in time to watch the spider rub its belly in satisfaction. Other than catching the spider in the act of biting, black widow bites are diagnosed through a bit of detective work.
Latrodectism is the medical term for black widow spider envenomation. Doctors have to diagnose black widow bites by asking patients when they started to feel the spider bite symptoms, how they discovered their bites and whether they saw the spider. Black widow spider bites lead to severe pain and muscle cramping, especially in the abdomen and back.
Folks often think any skin lesion is a spider bite (see our spider bite pictures for examples of spider bites and skin infections). Most of the time, these lesions are bacterial infections like MRSA or streptococcus. Black widow bites don't look the same as these skin lesions. Sometimes, a black widow bite will look like two small holes. There will probably be some swelling and redness at the area of the bite.
Black widow spider bites are rarely fatal. They can be treated in the emergency room with IV infusions of calcium and narcotic pain relievers, as well as antivenin for severe reactions. It sounds like this reader's experience was pretty typical. I'm glad he's alright.
Timms, P.K., and R.B. Gibbons. "Latrodectism - Effects of the Black Widow Spider Bite." West J Med. Mar 1986.