On a weapon with the blast quality and the stun setting (such as the ACP Array Gun from DC), would switching the stun setting on make the blast damage be stun as well? Or would it negate the blast effect?

I don't know the RAW answer but sounds fine to me.

I don't see any reason why not. That's how I'm planning on using my blaster carbine once I put a spread barrel on it.

Sam gave a related answer in that Pierce applied to the Blast effect of a missile warhead, it would seem to me it carries an Array Gun set on Stun would have that apply to its Blast effect.

My group includes a Bounty Hunter who carries (among other things) a blaster carbine with the spread barrel attachment. While it hasn't come up yet, I have every intention of letting the Stun setting quality apply to Blast as well. I see no reason why it wouldn't.

As blast damage seems to carry over other special rules from the weapon I see no reason for stun not to as well.

Otherwise, Stun grenades would be pretty pointless.