The game should now open in a borderless window. Once inside, re-save your graphics settings to overwrite the file correctly.
This is the oldest trick in the book for Ubisoft games. The black screen often occurs because the game is stuck in a broken fullscreen state. assassin 39-s creed rogue black screen fix
Steam\steamapps\common\Assassin's Creed Rogue\ The game should now open in a borderless window