Spider bites are harmless, however, in the case of the black widow spider, the bite can be dangerous. In some cases, after the spider bites, blisters may appear on the skin. If proper treatment is not given, it can lead to scarring and skin damage. Spiders such as the brown recluse and the black widow are poisonous and their venom causes a severe allergic reaction that can rapidly spread throughout the body. The person may even die, if he is not treated carefully.