Most of the time blondies will sink when they are too raw in the middle. This can be either a result of undercooking or, more commonly, it can also happen when the eggs and sugars are not mixed together well enough. In fact, under-aeration of the blondie batter will result in a doughy, greasy mess. For more information, check out this great article on, "Why Your Blondies Are Raw In the Middle" over at Food52.