Here's a neat trick you can use to encounter the Black Mushrooms at will:



Go to the "Final Rest" area of End of the World. Save your game, then do a soft reset. Load up the game you just saved. Then, go into the room with all the Heartless. There should be some type of mushroom there now (black, white, and the annoying bouncy ones).

Anyway, if you don't get the mushroom you want, or you don't encounter them, save, then soft reset again.

Now when you do encounter white mushrooms, you should use the same spell to please them each time. You'll have a better chance of getting the Mystery Goo if you do, plus you'll get a Magic Arts and a gem, which is nice if you don't have Save the Queen yet.

Just for reference, here's the spells you have to use depending on the action the mushroom takes:

Light over its head- Thunder

Floating- Gravity

Shivering- Fire

Fanning itself- Blizzard

Spinning around- Aero

Stops movement- Stop

on the ground- cure

Make sure you use the spell while they're doing these things, and not after. And make sure you don't take too long, otherwise they'll run away. Well, good luck!