As to how you know Oda hadn't finished his design, not sure where you sourced that information from.
- Who else's design didn't he finish, did he say this?
- Is this from the SBS or a magazine?
I'd like to see where you read this from as official fact.
It is still canon if you can't give an official source, so we can't say he doesn't use guns especially being roughly 20 years after God Valley.
Chapter 0 is pretty UpToDate, it's 2 years or so before he became Pirate King and 3 years before he died. Hard to say he wasn't also a sharpshooter, or was not solely a Swordsman in other words as these are in the final moments of his life.
On top of that, he was said to have the strongest Haki, so it doesn't appear to me that Haki is the main thing that determines a Black blade. Looks like there's more to a Black Blade than just Haki, and Swordsmanship is possibly also a criteria.