Only three characters are known to have forged a black blade: Ryuma, Mihawk and, in the future, Zoro.
These three share a common trait: they are all dai-kengou who aimed at the top of swordsmanship and therefore spent their lives actively looking for adversaries to duel (this is more important than many of you seem to believe: the idea of dueling your way to achievement is something with a distinctive weight in Japanese culture and we see it reflected in many popular works; for example, winning a certain number of fights as an additional requirement to "evolve" is a mechanic found in games such as Digimon, Final Fantasy or Pokémon). No other character you will think of share these two traits: most if not all of the strongest swordsmen you will think about (Oden, Rayleigh, Roger, Shanks...) have other ambitions and aren't even considered dai-kengou in spite of weaker characters (Vista, Ushimaru, current Zoro) being recognized as such.
So far it seems obvious that those two are main requirements for a black blade and yes, it probably has something to do with becoming one with your sword in a deep level that only the greatest swordsmen (which isn't the same as strongest swordsmen) who aim at the top of swordsmanship (cutting anything but nothing) can achieve.