I suppose the most vast improvement would come from electronic advances. With digital recording devices for voice and flight data, the quality and quantity of recordings will have increased tremendously to match the more advanced aircraft in use today (processing and storing all the data from current flight computers is a tall order). The engineering for the crash-resistant box in which the sensitive electronics reside may also have improved with the advent of new fire-retardant materials and better structural alloys with which to make the housing. The purpose, however, remains unchanged. They are still used to find out what went wrong in a crash.