Harder than Secret Rings, but just as good, too bad the main story is so short.
First, we shall take a look at the swordplay. If you've ever played The Legend of Zelda: Twilight Princess, this mechanic will feel familiar to you, swinging wiimote= swinging sword, but most of the swings do the same thing, except in the case of a combo, aerial spin, or a special move, known as 'soul sourge.' Even so, it still fills very good to slash through many enemies while going at 100 MPH.
Next, the graphics. The visuals in Secret Rings were very good, and Black Knight's are an upgrade from them. They aren't as cartoony and colorful you'd expect Sonic's world to be, but this isn't Sonic's world, it's King Arthur's, and they are great, especially the opening scene, and the other two CG scenes after words, can you say, PS3 graphics? The in-game engine is still very good too, very detailed, very lush.
The one good-bad part is the high level of difficulty in the later levels and bosses. The levels get at least twice as hard with each level, same with the bosses, although the final boss is pretty easy once you get used to it, as are the earlier ones. But some of the levels, can make you want to rip out your hair.
Finally, the minor flaw of this game, the short story, in a few words, short, fun missions, not that many missions, and a 6-8 hour game really.
Controls:[8/10]
Graphics:[9/10}
GamePlay:[7/10]
Story:[6/10]
Final: 8.5/10
Also, here's the key to the rating system(in order of importance).
Number of times hit
Number of enemies slayed
Amount of rings
Finish time
EDIT: Sorry, that rating system key is more for the bosses.