From what i've seen, there's no actual black wind naginata but let me break down the verified legendary options that'll give you similar effectiveness. The shade of blight is probably your best starting point, found during the kurai eikyou quest when you face the messenger - watch for his poison-enhanced attacks before going for the kill. The vengeful foe, which you'll get from the tournament questline, has this really nice bleed effect that triggers on perfect parries, and the timing window is actually pretty forgiving.

Looking for something with more range? The sage's reach becomes available during showdown in sakamoto, but you'll need to position yourself carefully when taking down the naginata since they love to counter aggressive approaches. The time-honored crescent over in kashiwabara fortress is another solid pick, just remember to clear the outer patrol routes before attempting the inner courtyard. The celestial blade from the yuki no kata fight requires really precise dodge timing, especially during their spinning combo, but the damage output is worth the effort. The lustrous pearl from the yamabushi quest chain has this amazing sweep attack that's perfect for crowd control, just make sure you've got enough stamina before committing to the full combo string.