I agree about black, but blue is fine at 2 CMC. Think twice is excellent, and fits with many blue strategies or at least gets you there, as is couriers capsule. I'm not usually having trouble at 2 CMC for blue, and while 4 is a bit thinner, there are still some excellent option in both blue and black, you just have to build around them.
That said, I've noticed a lot of my decks skipping the 4 slot, so I might be wrong there.
;-)
I'm late to the party but still wanted to touch base on this. Blue is not fine on curve at 2CMC. There are good blue cards that cost 2CMC but a very select few you would actually play on T2. The most important note though is that none of these good blue 2 CMC cards are board presence. Sames goes for Black. Now
in Tidehollow or other
options can be brought up but those costs can be problems to play on curve (generally I'd like to see a
in a mono black or very heavily black themed deck with late color splashes). So outside of those cases with restrictive costs, not much is hitting the board until T3 in those colors (and makes Dimir's options even worse).
The noticing of the shallow 4 CMC slot in your decks is the indicator I was mentioning. Folks did it anyway and mentioned 'what about this 4 mana CMC card?!?!' but if you do a comparison of cards at each point on the curve, 4 is the one slot with the smallest amount of "good" options (or options that get played regularly).
elk