The wiki description for the Sacrificial Dagger is:
Dagger orbits Isaac, very high damage.
Does it also have the projectile blocking property of the other orbitals(i.e. Meat Boy or Halo of Flies)?
The Sacrificial Dagger does not block projectiles. I tested this with an orange fire, a Sacrificial Dagger, and no other orbiting items. I watched the projectile pass right through the dagger several times.