It's just a matter of energy RK is represented by a black rose (which is also transformed into the petals of a black rose when it moves in an instant). RK doesn't use black roses to fight Ruiqi, but black roses represent things like wands and magic sticks. RK uses a imprisoned consciousness array (see video time at 09:25 for details, RK writes: "Wake up and imprison the consciousness array." Just like Ruiqi's sword of inheritance, black roses are just the sustenance of RK energy.