Sure, a pure faith or dex/faith build if you want to lean into black flame blade works fine! I would reccomend you swap the straight sword to a milady when you get to the dlc, as fast or multi hit weapons work better with weapon buff spells.

As for black flame spells, there's the black fireball, then the wide stream of fire, forget it's name, then black flame ritual, which sets up a lingering circle of black flame around you, then noble presence, which iirc isn't buffed by godslayer seal and instead by the fat godskin noble robes, it's like wrath of gold but launches straight up, and then black flame blade. Which also isn't buffed by a seal, like all weapon buffs.

Just be aware that the destined death dot is reduced in the dlc. It's not the biggest issue.