You have to think about Bitcoin like thinking about Gold.
You can only Buy it if someones is willing to sell gold.
And the Blockchain can be seen as a big book with all Transactions in it.
So if you buy a BTC the Blockchain memorizes your Transaction and you get your bought BTC's from a Seller
I am not trying to be that pedantic guy, but it IS really important that we start saying "blockchain.info" or BCI… or more and more people will start to think that
THE Block chain is selling bitcoins to users
Even though Blockchain.info can be used to monitor block chain activity, it is important to make the necessary distinction between the two. Blockchain.info is a web wallet and a platform (block explorer) that serves as a visual representation of the Bitcoin block chain.
Clearly, OP is asking about Blockchain.info services so it would be good to keep the discussion within that topic.