Every barrel I've used up until this BA 300BLK barrel has been dimpled for the set screws. What is the best way to make sure the block is aligned properly without having those for reference? I'm sure this is a lot more simple than I'm making it.

Take a credit or business card, line up and make a mark at the centerline of the barrels gas port. Then make a similar mark on the centerline of the gas blocks port. Line up marks as you install the block and you're GTG.

Thanks, good idea. Is there a need to leave any gap against the "shoulder" or just snug the block up to it?

here's a method I read somewhere and have used with success:
take a matchstick (wooden) that fits snug but slides freely in your gas port, stick it in the gas port and break it off even/clean with the surface of the barrel. Slide block on and turn the barrel over, move the block around until the matchstick drops into the block. Turn it back over and make sure matchstick drops back into barrel, tighten block again, make sure matchstick still moves freely with gravity, then stick a cleaning rod down the barrel and break the matchstick off, dump both pieces out the barrel. As a final check spray brakleen down the gas tube from the receiver end as part of cleaning before firing.

Every barrel I have has about a .03" gap from the shoulder. I use a pair of calipers to measure the location and do the math to center it.

There are many methods to get you there, here's the one I use. It relies on the fact machinists are lazy, or efficient, however you want to look at it. The easiest way to drill the gas port on the gas block is to do so through the set screw hole so those should line up.

Remove the set screw completely from the gas block. Check that the gas port in the block is centered with the set screw hole. Slide gas block onto the barrel so the set screw hole closest the journal shoulder is centered over the gas port. Make note of any gap between gas block face and journal shoulder. If there is one, check the gap with a feeler gauge. With the feeler gauge in place, rotate the gas block 180 degrees, reinstall and tighten set screws.

so that's how they get that blind hole in there!

I always snug the block against the shoulder. Never had an issue with reliability.

That was a tongue in cheek jab, glad someone got it...

Sure as hell..... there's only an ever so slight gap between block and shoulder to get the set screw aligned perfectly.

Most will have a gap when properly aligned. I have had clients bring me a mixture of off brand parts over the years and have found a few combinations that dont have a gap when installed.

It's also worth noting that on a lot of full gas guns, the alignment really isn't that crucial to function. However, add a can, adjustable gas block, etc and it starts getting more important.