Blue drake will be a rare drop from Maly-10, similar to the Azure drake from Maly-25.

Also, Blizzard seems to have changed it's mind on the more "common" achievement mount drops. They have left in the drakes for the Ulduar 10 and 25 and Glory of the Hero, even though they are all relatively easy to get now. At the very least, they will give plenty of advance notice before they are removed. Since Cataclysm is a week away, I doubt they plan to remove any more mounts other than the ones listed above.