If you get the new version, the Alchemist now tells you where to find it with more clarity, and also, when you get there, it's much easier to spot the secret!
I got it! But now I'm in the blue cucumber room, before the 30 blues door-and when I use the shell on the shell in the background here, there's a sound effect that plays over and over very quickly without anything else happening. (If that's supposed to get you a blue cucumber, I'm at 29/30.)
e: found 30th one, I missed the one in the PC house!