To discover black areas in the map you don't need to do any quests, you just need a small talk with someone.

Guide

Open Map (M in PC) and you will see some undiscovered areas are completely black and their details can't be seen.

1. To open those areas you need to go to the fire icon on each zone.

2. When you get there a guy named Wayfarer is waiting for you.

3. Talk with him and after a small chat that area on the map will be discovered.