Go to your bag, go to Key Items, and then click it to open it. If you have made pokeblocks in the contest hall, you can feed one to your pokemon.
If you are in the safari zone, walk up to a white platform, press A, and you will have the option of putting a pokeblock on to attract wild pokemon.
Chat with our AI personalities
Go to the contest hall in lilycove and talk to the lady on the left. She would give you a poke block case.
You can get it in Verdanturf city in the Pokemon contest.
You buy them in the Pokemon markets.
I don't know about ruby or sapphire as i don't have them, but i do know in emerald you get the pokeblock case in Lilycove by talking to one of the ladies behind the counter in the contest hall.
Green!