It could have something to do with the fact that higher level gathering skills yield better and more frequent blue recipes?
I don't know, I'm just speculating.
It doesn't appear more frequent (however I have not admittedly been checking). Also, I now to seem to pull mostly purples, I actually don't recall the last blue I got. Given the total disinterest on the AH though they're still vendor trash regardless. Interesting idea however, I hadn't really considered it might be an intentional decrease.