i have had some success doing overnight mashes, though admittedly it's been easily 20 years since the last once. i always added some extra cara-pils to the recipe and I never had any off flavors and, as long as I kept tge temp up in the 145-155, great results. With a dark beer I would mash the base grains overnight then add the dark grains the next morning. i'm not exactly sure what the effect of the pH will be for the overnight mash but you might add some sodium bicarbonate to the mash when you add the dark malts after a pH check.