I think Vale of Eternal Blossoms is where I caught my Tiny Blue Carp, but check out El's archived page on catching the Tiny Carp. There are a couple of other places where the blue one can be fished up.
Feel free to browse through Perks N Peeves, my vanity pet collecting blog!
The most recent tiny blue carp I fished up was in the pool above Guo-lai Halls in Vale of Eternal Blossoms, that was after the destruction of the vale. I think they can drop from the pond on Timeless Isle too.