Dogs have 2 types of hair; Overcoat and Undercoat. Sable dogs have over and under coat of different color. The overcoat is of usually of black color and you can observe where the black is going to eventually develop by observing the black hue on older pups. The undercoat is of different color like red, brown, grey sable and so on. Please keep in mind that there is no such thing as black sable that would mean that over and undercoat would be both black and thus such dog is SOLID BLACK.
Please keep in mind that sable pups that get born dark, that is as dark as they will be eventually. Than they will get lighter as they mature since at that time all you see is the undercoat. Pups will get darker as they mature and black overcoat will appear .That will happen around 1-3(!!!) years. In order to estimate the color of your pup you must look at the parents and grandparents and not at the pup since there you can see only the undercoat (puppy coat) .
Please also keep in mind that color is not important and that GSD should be selected by temperament and structure and not by color.
Below is the same dog to show the color progression overtime.