for the BLAST blasphemy, it says that the strength of the ability scales with CAT. How exactly does that work? Do I roll more dice at higher CAT?
I dont believe there are damage dice in this game. The CAT scaling I believe changes the dice roll itself. A CAT1 blast affecting a CAT 4 sin? Well, it might make it so the roll is hard on top of being risky, or even impossible unless the group finds some weakness. But a CAT 4 blast against a CAT 4 sin will still be risky but wont be hard.
You do roll more dice at higher CAT. Blast is a PSYCHE roll, and psyche is a special action score equal to half your CAT (rounded up).
Because each success counts as one slash when you're acting against a talisman (including the sin's execution talisman), you would be dealing more damage on average the higher your CAT (and therefore psyche) is.