This is going to be hard to answer, because it would require the person answering to have played both, and I know the likelihood of finding someone like thati s low unless they love the Hell out of Ninja Gaiden, like I do.
Now in Ninja Gaiden Black, enemies would be replaced by more difficult ones - from later on in the game - when you played on higher difficulties. Like on Normal, the first enemy you meet is a normal Brown Ninja. In Hard, a white Ninja Captain. In Very Hard, the dreaded Black Fire Ninja. In Master Ninja Mode, it was the Red Fire Ninja of all the horrible things.
More than that, though, once you got up to where the highest-tiered enemies were supposed to be, they would be replaced by new enemies altogether. Like those red dinosaur fiends? They'd be replaced by tremendous skull-wearing Ogre demons. I think those guys were originally in the Hurricane Pack. Shit, they replace a later boss with a much, much more horrible one. Then they start adding in bad-ass normal enemies to the boss fights! Things got well and truly crazy.
Does Ninja Gaiden Sigma retain these horrible but also awesome features? It may be relevant before too long.
