Blue is difficult because for Impervious you really only Behemoth and Plague (I only got that trait for Plague today too).
If you need to stun for like Famine, unless you have Stonehammer (which I don't) you need Emperor Korvash with his third trait (because Famine will almost always be in the 3rd or 4th slot.)
If you don't have those guys, then you need to be about speed (low cost spells), true damage and mana drain. Morthani's Will is great in the first spot. Astral Spirit is a great troop (and has a double bonus this week). If they have something deadly in the last spot, Dragonian Rogue is great as he hits the last spot and destroyed gems help fill the team. If you need the Dragonian Rogue though, than you probably need Knight Coronet up front, so Astral Spirit also has to come out.
I personally plan to start with Morthani's Will, Astral Spirit, Crimson Bat, Queen Ysabelle as a base. Will adjust from there.