I have wallet on blockchain.info, with two addresses. I want to send some bitcoins from only one of them using quick send. I don't see where I can select source (input) address. Where I can see my default (master?) address? How can i change it?
The Blockchain.info's My Wallet "Quick Send" feature just selects one of your addresses that has a sufficient balance to fulfil your payment. If none has, it will combine multiple to fulfil the payment amount.
How Blockchain.info choses the address is not specified. My best guess is that it will try the oldest output first. Keep in mind that it is not possible to always send from the same address since you will most probably receive your bitcoins on multiple addresses.
If you want to specify from what address to send, you can use the "Custom" option. It will let you configure your transaction from input addresses to change addresses. It also supports multiple output addresses.