Assuming that your docks are still alive a month after your posting :)and you still have not found the problem - They are known to be moist soil lovers and to wilt severely in the heat and drought... though timely watering will help it bounce back (like impatiens do)
and they should be divided every 3 yrs to reinvigorate the plants and keep them from dying out. that means that they are really considered "short lived perennials" though most nurseries fail to point that out. I dont know if any of this info covers your black leaf problem but its possible that its a combo of the two with the above normal temps.