Some things can't be fixed by blocking, but many can. You should completely wet the blanket, then lay it on a large towel (or another blanket if a towel isn't large enough). Then roll the towel and baby blanket up together. Press down all over the towel to remove as much moisture as you can, then lay the blanket on top of another dry towel and stretch the blanket to the proportions your want and leave it to dry. If, after the first washing, it reverts to the saggy baggy middle, you may want to consider steam blocking the blanket. If there aren't any large textured stitches, like popcorn, you can spray a section of the blanket, then lay a towel over that section and use your steam iron to set it. Be very careful not to actually touch the blanket with the iron. Repeat with more sections of the blanket. This will "kill" the piece. Once you have set it this way, it will retain that shape.