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.
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you can't but you can make poffins Poffins in diamond and pearl replace what used to be pokeblock.
If you are playing on emerald, you can interact with any of the tables the npcs are sitting at to make a pokeblock!
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.
Mix a Pecha berry :P
Liteblue pokeblock..