You can not 'get' this item, you can only use it. It is found at the contest hall in Lilicove City, and they are not found anywhere else. (Except maybe other contest halls)
you get it from your rivil
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.
pokeblock case
A little girl will give it to you in the slateport pokemon contest room
you can get yourself a pokéblock case at the lilycove contest hall.
You can't. When you have altered a pokemon's condition with pokeblocks the change is irreversible.
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.
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.
in what Pokemon? in Pokemon ruby/sapphire and maybe emerald too, a girl in slateport contest house give it to you
You can get a few at the Pokemon safari just outside of Lilycove. You'll need pokeblock though, which you can find at the Slateport contest hall.
The Pokeblock Case can be obtained from the person behind the counter at the Lilicove Contest Hall.
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.