Why in this build guide using a keen over flame art affinity?

[Padoru]

https://www.youtube.com/watch?v=_5SZl3BhvPw

For context, it is a build surrounding in blackflame blade/bloodflame blade using godslayer seal and uchi (has an innate bleed) as the weapon.

Based on my understanding, godslayer seal and its incantation spells scale on faith, so higher faith means higher damage using blackflame blade and blackflame incantations. This means that giving a faith affinity on the uchi, it will deal more damage right?

But I wonder, why in the guide, it uses a keen affinity over flame art affinity?

Im still fairly new in the game so pls little to no spoilers. Thanks!

[Verdite]

The flame art or sacred (faith) affinities means that the blackflame and bloodflame blade spells can't be applied to the weapon.

[Padoru]

Originally posted by Verdite:

The flame art or sacred (faith) affinities means that the blackflame and bloodflame blade spells can't be applied to the weapon.

I see, thanks for answering. I havent obtained the whetstone yet so I didnt know that.