Here are 2 enemies that can never fight. Both cards were designed in order to counteract the other one. How do they stand up? Actually fairly well, even in comparison to newer cards. For double-white cards, there aren't that many better cards than White Knight for that cost, but it's not the best (better cards include Auriok Champion, Elite Inquisitor, and Samurai of the Pale Curtain). Black Knight fares about the same with other cards that cost double-black (Bloodghast, Kalastria Highborn, and Nantuko Shade). But, if we ignore any comparison to all other cards, how good are these two on their own? Pretty good. A 2/2 for 2 with protection and an ability? Sounds pretty solid to me! However, the biggest stumbling block is the double mana cost, which makes it a little harder than usual to splash. It doesn't render it impossible, it just makes it a little harder than say Brood of Cockroaches or Mystic Familiar.
Note: I'd like to point out that I find it weird that Black Knight has been printed 8 times whereas White Knight has been printed 10 times. I'm not inferring racism, I just think it's odd.
Pros: 2/2 with protection and First Strike for 2
Cons: Double mana cost