Are you 100% sure you are feeding at the correct temperature? Using a reliable thermometer to test the temp? 40 degrees Celsius is fine and not over 42 degrees Celsius to be accurate and avoid too hot of a temp. Did you microwave the formula? If you did this could be a cause in an uneven temperature of the formula.
Crop burn is generally caused by feeding formula that is too hot (>110ºF or 43.3ºC) or less commonly, contact with a heat lamp or heating pad.
You need to keep an eye on this especially since a blister has formed... and hoping it doesn't lead to a hole in the crop which will present a very serious issue as formula can leak through that hole. Is this baby still taking formula? Are you feeding by syringe or how? Is there a heating pad under the container you are keeping the baby in?
This paragraph on crop burn is from the link attached below for you to read.
Crop burns occur when a bird is fed food that is too hot .This often occurs in handfed babies that are fed microwaved formula with "hot spots" due to improper mixing.The desired temperature ranges from 105-107 degrees Fahrenheit. Ideally, breeders measure the temperature with a digital thermometer, but if this is not available, breeders are using the inside of their wrist or squirt a drop of the formula on their lips to test the temperature. Eventually your wrist will desensitize to the warmth of the formula and you risk serious burns or feeding food that is too cool for the chick. However, the latter carries its risks, as human skin tends to desensitize to heat.
If detected early, flush the crop with cold water. If crop appears swollen and discolored, apply vitamin A and D ointment and feed small meals.
Several hours after feeding, the skin over the crop appears red. Owners may miss the early signs and only realize there is a problem when food or fluid leaks out of the crop onto the bird's feathers. The veterinarian will place the bird on antifungals and
antibiotics while waiting for the extent of the burn to become apparent. Surgical repair will be necessary to remove damaged tissue and close the crop and the overlying skin.
Crop burns occur when a bird is fed food that is too hot .This often occurs in handfed babies that are fed microwaved formula with "hot spots" due to improper mixing.
www.beautyofbirds.com