How is the difficulty of Demon's Souls/Dark Souls compared to a game like Ninja Gaiden Black?
Probably the same. They are both hard, but there are some key differences.
In Dark Souls you need more strategy, and also you need to have tremendous patience and execution. You need to pay attention to how the game works, stats, items, effects, it's an open-world RPG. Once you master how the game works, you master the game.
In Ninja Gaiden, while still needing strategy, patience and execution although not that much compared to DS, the most important thing you need is mad skills. reflexes and agility in using the controller. Once you master your reflexes and agility, you master the game.
imo I would probably say Dark Souls, It's more frustrating for me, died too many times from being too fast, or being too slow, or being too dumb or too smart, you just need to perfect it, You need to master everything in order to beat it,
Also there's a big penalty on dying, you lose all your souls(aka money) if you die again before you can retrieve them by going back to the place you died. also if you play multiplayer, it gets much real harder. Anyone can invade you and destroy your game in an instant at almost any point of your game. have a perfect plan for this part of the map? yey someone invades you and pwns your sorry little ass. Some people even turn off their console internet just to play DS peacefully.
The game is one big map too, and one big continuous event, one save only and it autosaves every x minutes, if you lets say accidentally kill an NPC 4 hours into the game, the game autosaves and that NPC is dead forever until you restart a new game
, so that's a bonus 
