This is a hard question to answer. The best Pokeblock would depend solely on what nature your Ralts has. The nature is what decides what pokeblocks the Pokemon would like and not the actual Pokemon type or species.
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Liteblue 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 one out of berries. It will be in your pokeblock case. Feed it to the Pokemon that you want to give the pokeblock to.
there isn't a white pokeblock
It depends on its nature, or just give what color it is. A brown pokeblock would do