How can I export a single wallet from bitcoind? On my full node bitcoin wallet, I have a lot of wallets, I can see them with the command dumpwallet "filename" but if I try to export one of those priv keys to another wallet, (blockchain) this New wallet service recognize the public address for this priv key, but not the ammount of bitcoins corresponding to this wallet... Am I doing something wrong?
To clarify, what you exported is a private key for an address. A wallet is a collection of addresses and their private keys.

The amount that you think is associated with that address is most definitely not the actual amount. What has most likely happened is that you have made a transaction in the past which spent some of those coins and the remainder has ended up in a change address. When you make a transaction, the change Bicoin always goes to a newly generated change address that is part of your wallet.