Mechanics might be driving requirements here: looks like this plane exists as mostly sea, with scattered islands. A good water-mage can control the sea.

I'm pretty sure the discussion of the flavor came first. We discussed blue being full of cephalopod monstrosities that thrive in a humongous sea. In the original discussion, it was either hinted or decided (I don't recall) that there was no truly intelligent life. There are certainly no humans.

Then again, the fact that the Gloamlings, as a whole, "want" the destruction of Aer as revenge hints at some sort of intelligence. Perhaps there is a guiding force for Blue, whether it be one great mage or a group of sentient creatures.

I'd just got done looking at a lot of the blue files. Originally, there was a giant monster theme in blue. When I submitted my own outline of cards, I had a soldier and a wizard (eels, acutually. I was still going for weird), but we discussed it and I downgraded them to Minions. Deep Ones style, I suppose. Later on in black we started using minions, so when I made my current pass, I traded them for a crab and a sponge.

That would seem fine if left alone, but we're currently having a mechanical identity issue with U/G, and it isn't helped much if both colors can also be described as 'mindless'. I think we clearly need two distinct cultures in these colors. Can we get something a little more than "theyv'e got a lot of tentacles"? I do like what's going on over in green, by the by.

Green can't be described as mindless anymore, not if they're actively worshiping a deity. I agree that the two need distinct cultures, and in fact, I agree that they need cultures period. The beings of the Gloaming can't really be angry or feel repressed if they don't have some sort of intelligence.
The more I think about it, the more I like the idea that blue is ruled by a species of some sort of sentient sea creatures. They are outnumbered by the non-intelligent life, but are powerful enough to bend those creatures to their whim.
I'll take suggestions on what type of creatures the "ruling class" of blue should be. I propose octopuses or squid.

Ugh, wait. I said all of that without thinking enough. Is it too similar to black now?
I would argue no, because the blue rulers aren't really enslaving, merely manipulating natural wildlife. Okay, let me change the flavor a bit. Hearkening back to cmeister's suggestion, the blue rulers are instead sea-mages able to manipulate the tides (flood) and the creatures that inhabit the water. Think Aquaman. That sounds sufficiently far from black, right?

Hmm... perhaps they're a little like how Rome used to feel. Like they were the only bastion of civilization, having conquered 'the world', and that everyone else outside their kingdom was just barbarians?

If they act like that, imagine how they'd react when they found out that they were just a colony of a much larger empire...

Mmm, sea-mages that manipulate tides are a good concept.

I'd like to establish what race (/races?) we're thinking these sea-mages would be. At the moment I think Aer is conceived to include vedalken and merfolk, but we don't have to. "Creature - Squid Wizard" sounds a bit too ridiculous to take seriously.

Huh. That does sound pretty ridiculous. What about cephalids?

Cephalids would seem to fit the sentient part rather well. I like it.