Oden was 39 when he died. He's used the same swords since childhood. He constantly got into fights his whole life. He sailed with two of the most prolific pirates in history. He had ample opportunity to turn his blade/blades black. The fact he didn't means he was fundamentally lacking somewhere to keep him from perma blackening his blade.
Recall that we were introduced to Mihawk when he was 41. Just two years older than when Oden died. He didn't own Yoru when he was a young man during Roger's execution, so he had that blade for even less time than Oden had his. And the way he spoke of Yoru to Zoro, it implies he's had it black for quite a while. And as far as we know, Mihawk wasn't born some unnatural beast like Oden. So if Mihawk could blacken Yoru in considerably less time than Oden owning Ame No Habakiri and Enma while Mihawk was not born some freak of nature, then there's no reason Oden could not have blacken his swords in all the years he's had them.
The fact Oden couldn't turn one of his swords into a black blade likely means he would've been incapable of it had he lived. Where he would have split his priorities between political matters and battles, as well as begin to plateau as a fighter. 39 is hardly a spring chicken.
