Beyond the knowledge of most people, the world contains a dark secret: of every person in the world, there exist two identical copies (called Doppeliners) with the same body and mind. Among the three, however, is distributed only a finite amount of fortune; should they ever encounter one another, the one with a smaller portion dies and the remainder is distributed until only one, the Master Root, survives. Keita Ibuki lives his day-to-day life as ignorant of this as everyone else until he stumbles upon a mysterious young girl named Kuro who explains to him these secrets of the world. Despite not wanting to have anything to do with her or the danger that inevitably seems to follow, their fates become intertwined and he finds that the Doppeliner system has more to do with his past than he realizes.
Six years ago, a catastrophic event ravaged Tokyo. Amaha Masune was found at ground zero, physically unscathed, but without memories and with a child in her arms. Returning to Tokyo for the first time since the disaster, the slightly clumsy but cheerful Masune just wants make a new life and to raise her daughter without interference from the Child Welfare Agency. But unsolved murders are being committed all over Tokyo and a near-miss reveals her identity as the bearer of the Witchblade, a mysterious weapon linked to the disaster 6 years before, that seems to have a mind of its own. Giving in to the Witchblade seems dangerous, but when the powerful Dohji Corporation agrees to guarantee her a life together with her daughter if she works for them, can Masune pass up the chance?
In both shows main character (female in Witchblade, male in Kurokami ) suddenly gets thrown into a world of mysterious powers and hidden truths. Due to various circumstances this hero/heroine also gets his hand on this power, and guess what - theirs one of the strongest (which obviously leads to the classical shounen dilemmas like "how to control it" or "how to make it stop sucking my life out of me" etc.) . In both animes there are evil mysterious organizations, that knows a lot about a those almighty powers and tries to artificially recreate them for their own malicious use. And main hero/heroine will obviously try to stop their plans from succeeding.
Both show aren't really the cream of the crop, they are the very mediocre shounen shows. Their level of amusement is very similar - fights aren't flashy enough to provide powerful eye-candy, and plot is... well.. it's shounen, what plot? But still, if you enjoyed watching one of those titles and need something similar check the other one as well.