A little girl will give it to you in the slateport pokemon contest room
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In Pokemon Ruby,Sapphire and Emerald after you make a pokeblock, you can check the pokeblock case and you give the desired Pokemon the pokeblock to help it do better in Pokemon contests. After you have received a pokeblock case and you made pokeblock at the berry blending machine go to your key items in your bag and scroll down to your pokeblock case then simply select the pokeblock you want to give your Pokemon.
you can get yourself a pokéblock case at the lilycove contest hall.
Make sure you have a pokeblock case and go in any contest hall and look for the blenders. Press A and you can berry blend, it creates one pokeblock.
The Pokeblock Case can be obtained from the person behind the counter at the Lilicove Contest Hall.
in what Pokemon? in Pokemon ruby/sapphire and maybe emerald too, a girl in slateport contest house give it to you