What you describe is probably "white druplet", a physiological disorder in blackberries and raspberries. White druplet is caused by sunlight, often following a change in temperature, such as a sudden sunny day following a humid cloudy day. White druplet is characterized by a change in color on some of the individual druplets on a larger berry. Some researchers have indicated that it is like a sunburn or sunscald, while others have thought it might also be linked to damage from insects such as stinkbugs. However, it is not a disease, should not get worse or spread, and will not affect the flavor of the berries. It is a bit curious though!