Extreme heat and bright sunlight can sunscald tomato fruit, leaving a light yellow to white sunken spot that resembles a blister. Eventually this area may allow black mold in invade and cause the tomato to rot.
Sunscald most often happens to fruit that is exposed to full sun after losing foliage to disease, hail or tomato hornworms. Exposed fruit may be shaded with cheesecloth to prevent injury. Fruit can also be harvested as the tomato starts to turn color so they can ripen inside. Tomatoes picked at this stage will be just as sweet as those left to ripen on the vine. Remove affected fruit to encourage more fruit set.
Sunburned fruit are rarely usable if the damage is extensive. Tomatoes with little damage can be used if sunscalded areas are cut out. (Ward Upham)