Old Blue (also known as Test Blue) is a color, featured only in Penguin Chat 3. It is not part of Club Penguin, but can appear as a result of some glitches. Some images of penguins in the game also use Old Blue.
In the game Catchin' Waves, if you play in Survival Mode, in the short intro that is shown, right before your penguin is lifted up by the penguin on the jet pack, two Old Blue penguins are shown behind your penguin, and one is holding a red surfboard.
Contrary to popular belief, Old Blue was not an obtainable color in Penguin Chat 3. Like it is in Club Penguin, it was also a test color, you would turn Old Blue in Penguin Chat 3 when dancing and using the construction hat.
Old Blue is very old, as it was the only color for Experimental Penguins and Penguin Chat, and it was in Penguin Chat 3, and it is still in Club Penguin to this day in various glitches, tutorials, etc.