Hi Ace,
This is likely to be a genuine blade by Onizuka Yoshikuni who was an upper quality smith working in Chikugo Province in the early 1600 hundreds. He may have studied with Hizen Tadayoshi school prior to moving to Chikugo. Mostly worked in suguha. He is listed in most of the usual sword reference books. Nice blade you have there, hope it polishes up well for you.
Cheers
Steve J