It depends on it's nature(personality) or if you're not sure, just give it yellow pokeblocks which is sour pokeblocks
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Mix one out of berries. It will be in your pokeblock case. Feed it to the Pokemon that you want to give the pokeblock to.
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.
in order to get the pokeblock case, you have to go to the Pokemon contest in lilicove.After that, talk to the girl on the upper, upper, left with blue hair.talk to the one on the left.then she will give you a pokeblock case.thank you for reading.hope it helped =]
Pokeblock's are not in diamond instead there called poffin's, just like ruby, sapphire and emerald in the Pokemon contests you can create your own poffin using berries.
go to the contest hall and ask the lady on the left hope i helped from Aleandro