When refusing Kyurem and Zekrom/Reshiram together, Black/White Kyurem's Level, Nature, EV's, IV's, Shininess, Characristics, Stats and other traits are all determined on what Kyurem's is. The only thing that isn't is it's Ability which is the same as Zekrom's/Reshiram's.
In the games, Kyurem changes its form through the use of the DNA Splicers, which fuses it with Reshiram or Zekrom. The combination retains Kyurem's Poké Ball and held item, but assigns Reshiram's Fusion Flare and Turboblaze to White Kyurem, or Zekrom's Fusion Bolt and Teravolt to Black Kyurem. This fusion is reversible, with Reshiram or Zekrom remaining unaltered from before being fused, including whatever item it may have been holding. Kyurem has a different cry in each of its forms, with White Kyurem's cry being an alteration of Reshiram's cry and Black Kyurem's cry being an alteration of Zekrom's cry. Both cries have Kyurem's cry in them as well.