Do blast weapons always hit the body location? I know Spray weapons do.

Always body, as far as I know.

I can't cite a source, but AFAIK, it's always Body.

In Deathwatch you roll hit location and damage individually for each person affected by the blast, but they changed it in Black Crusade and Only War, so that you only roll damage once and apply it to all affected but don't say anything about hit location.

Short answer:

Blast weapons can be restricted to Body hits without disrupting gameplay.

Long answer:

My issue with Blast only hitting the Body location is that it devalues every other location's Flak armour. Throughout the books, Flak increases location AP by 1 as long as the Blast isn't a direct hit. However, reasonably attainable Carapace armour is stronger than Flak even with that bonus, so defending the Flak perk is admittedly moot. Rolling for each target's hit location would make more narrative sense, but easily eats up valuable time when a Blast (8) weapon is blowing up thirty people. So yeah. Basically just Blast attacks to the body is close enough unless only one or two people are getting hit.