"Overpriced" does not refer to "points"
Witchfire: Harness warp charges, roll to hit, opponent can deny and add in adamant will, psyker, higher ML
Blessing: Harness warp charges, opponent can deny on a 6
Witchfires tend to have a weapon profile, like 24" large blast, ignore cover S4 AP5 2WC
Blessings enhance a unit and give various boni like ignore cover 1WC
In the end the blessing will always win out as it is harder to deny, doesnt need to hit and will do a lot more than the witchfire power.
That is why lores that revolve around witchfires are mostly ignored while lores that heavily rely on blessings are prefered.
A unit of centurions is much more powerful with a 4++ and ignore cover than a psyker that can cast 2 large blasts at S4.