Purple blisters can be a sign of a flesh-eating disease from raw shellfish. According to the CDC, there are other signs of necrotizing fasciitis that you should know, because symptoms of the disease often spread quickly and can be confusing. Common signs of necrotizing fasciitis include a red, warm, or swollen area of skin that spreads quickly; severe pain; and fever. The agency urges you to seek medical attention immediately if you notice these symptoms, especially if you've recently had surgery or an injury.
The later symptoms of necrotizing fasciitis are a bit more gruesome, and can include "ulcers, blisters, or black spots on the skin, changes in the color of the skin, pus or oozing from the infected area, dizziness, fatigue, and diarrhea or nausea."
There are multiple bacteria that can cause necrotizing fasciitis, but the CDC says most people get infected with Vibrio bacteria specifically after eating raw or undercooked shellfish, most commonly oysters.
However, certain Vibrio species can also cause a skin infection if an open wound is exposed to salt or brackish water. It's also possible for the bacteria to lead to an infection if raw seafood or raw seafood juices come in contact with an open cut.
To reduce the chances of getting infected with Vibrio bacteria, the CDC suggests avoiding raw or undercooked shellfish. Additionally, "if you have a wound (including from a recent surgery, piercing, or tattoo), avoid contact with saltwater or brackish water or cover the wound with a waterproof bandage."