As written, sacrificial blast seems almost too good, especially at high levels. There's a paladin prayer that requires you to sacrifice a healing surge to deal 4[W] damage, and it's a 5th level daily. I think it might be more balanced if:
1. It required the warlock to expend his second wind, which limits it to 1/encounter and leaves him dependent on other characters for healing if he gets badly injured; and
2. It deals extra damage equal to 1d10 per tier.
EDIT: I have similar feats on my blog here, but they're for my own created powers for my Holy Paladin specialized class here and here.
Your mention of the paladin power help me put this into perspective and I've taken your suggestion. Using a second wind was too much when I could just put into the description of the feat It could be used once per encounter.