As you only have to reach Mom, the challenge shouldn't be too difficult. Devil Deals can be potentially more dangerous than normal unless you are confident in your HP status. As for Curse of the Unknown, remember that you start with 3 HP as Isaac and simply try to keep track of any hits or heals you get as you proceed through the floor. If you want to make the Curse of Darkness less of an issue you can turn up the games gamma setting in options.
The second issue you have is trying to keep track of your HP as Curse of the Unknown will stop you from being able to see how much HP you have left at any time. The difficulty of the challenge comes with the screen being permanently darkened due to the Curse of Darkness being in effect. Your goal is to make it to Depths II (or its equivalent) and defeat Mom. Pitch Black is the first challenge and as such is not too difficult.
Completing all challenges (both base game and DLC) are required to later unlock Real Platinum God and 1001%.
