When can I use the output of that transaction: when the transaction will be confirmed at least once or when it'll be in the blockchain?

First, if that transaction is confirmed once, that means it's in the latest block, hence in the blockchain, so the choices you present are essentially the same thing.

Second, you can actually spend the transaction output right away. Miners may include both your original transaction and the dependent transaction in a block, which means you would have the same number of confirmations for both transactions. Whether your wallet software allows you to do that is another matter entirely.