Blade and Buffy must both fight an onslaught of vampires (a mixed bag of Marvel vampires and Whedon vampires) while also fighting one another. So there are two elements to this: who can kill the vampires better (both their own and each other's) and who would physically win when fighting one-on-one?
Blade with silver katana and silver glaive.
Buffy with mystical slayer scythe.
Edit: Buffy, prior to the comics giving her Superman's powers. That is probably temporary and does not reflect the character's origin or 18 years of history prior to it.
Blade after gaining his Dhampir/Daywalker powers.
This puts them on even ground and reflects the last decade and a half of continuity, respectively.
1: Buffy vs Blade in competition: who kills more vamps?
2: Buffy vs Blade in combat: who wins when turned against one another?