Leave them open.
That's the only way that any water that may get inside the ribs will be able to get out.
Though it may seem strange to poke holes in the ribs to purposely allow water in in the first place, it's a fact that most all bbl assemblys have at least a small area
of leakage along the feet of soldered seam of rib and bbl.
It's a rarity that there is absolutely no break in the soldered joint anywhere.

If the drain/weep holes are not there, that water that does get in there by those small voids, and it will,, will not smoothly exit the assembly as you are bluing.

It will start to seep out of one of those small cracks or voids as the water builds up inside and you handle the assembly after taking it out of the water.
Then that crappy water from inside the ribs leaks over the new blue and spoils the finish.

With the drain holes, you can hold and tip the bbl assembly as you pull it from the tank and let the water drain out w/o running all over the new finish.

Plus after the last cycle, put the bbls on a stout peg in one of the chambers and mount the assembly nearly upright.
Let the muzzle tip forward a little and the bottom of the bbls where the rear drain hole is will be in position to allow any water still inside to drain out and down the hole that the extractor shaft
sits in.
At this point take a propane torch and carefully warm the entire assembly to dry up any trapped water.
You will see the steam start to chug out the front and rear drain holes as well as extra water 'spit' from the rear/lower one.
Keep a paper towel handy to clean up any that comes out.
Keep the heat up untill you are satisfied that it's dry inside there.

Do not over heat as you can melt the soft solder. But you have to get to 400F+/- to do that.
If you are getting close to that you will hear the bbls start to make a creaking noise. That's the metal expanding.
You only need 212F to boil out the water.

Let it cool on it's own when done.

That's what I do anyway.
Others have different ways and it's the final results that count.
The worst thing is to have a completed, finished & assembled gun in hand and have some water leak out from betw the ribs.
I've seen that many times.