In regards to painting, if your color scheme is going to be black and blue, it's probably wisest to prime black. Honestly, priming white would be a mistake, black is black over any basecoat, there is no in between. The only difference would be the blue, but it would be more of a pain to deal with the white underneath.
As for your airforce, I'd recommend going with a uniform scheme for all the aircraft, and different squad markings for each type of unit. I think this would provide more cohesion to the force rather than a different color scheme for each, but thats my opinion. I like the scheme you came up with by the way
gray color scheme with blue as their squad color
But like I said, apply this to the whole force, not just one unit. Differentiate by by smaller things rather the whole color scheme.
I had a conversation with a friend of mine a couple years back that change my out look on color scheme's for 40k armies. To sum up what he said is forget that 40k is the dark future and think of it as the bright and shiny future. He has won multiple best painted armies at GT's and other events and every single one of his armies has white or bleach bone as a primary color and little to no black.
It is unfortunate that painting has come to this, and pathetic to say the least. It really only goes to show the lack of intelligence people have when it comes to judging the quality of models. The fact that people have to be won over by the mere presence of "bright and shiny" colors is ridiculous. Attention grabbing and well painted are not always synchronous, and very rarely are they ever in this case. I also find this disgraceful to those who should win best painted but have it stolen from them, because of the "bright and shiny" syndrome which seems to be a plague in competitive armies.
So my advice to you Steve, painting wise, is to do what you like, not what will get you points based upon the simplicity of other people. Your skills as a painter are not always reflected in your scores, as I and others have found out. If you can paint well to begin with, whether or not your army is bright or dark, the quality will be reflective of your ability either way.