Tbf Oda forgets to correct a lot of things. Someone made a compilation of his mistakes:
https://www.thegrandline.com/odamiss.html
https://www.thegrandline.com/odamiss.html
Furthemore, there are about zero chances that people, just like us, didn't notice the sword and didn't ask Oda in the SBS of the volume.
To summarize, I think It was too important of scene and too big of a datail to make a mistake twice. Also, there Is another scene too in which we can see the sword Is not white. It was way less clear, but still:
That should raise the number of errors to four, and all in the same sense (drawing the sword black and not white).