Hansli, the loading manuals will never give you such hints, but prematurely blackish hulls are caused by low pressure loads that don't seal the case mouth in the forcing cone on discharge. If you load very light 5/8 ounce loads, switch to one step faster powder to keep the pressure in line with good ignition. I would switch from Unique (which I use in 20 and 28) to Green Dot for 5/8 ounce loads. Good luck.