blockchain is not an exchange, you can't buy from it
blockchain.info is just a block explorer, they don't sell anything
since you are a newbie, the "downloaded an app" part might contain a virus, better be careful
I don't know if I would use exchanges through mobiles
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You will be buying it from blockchain.info - they are not only just a wallet, the people behind it are into various bitcoin/crypto currency projects.
Hmm these 2 posts seem to contradict each other. Who's is right?
As of the first reply, i got the official blockchain.info app from the App Store? Same as the link from the website.
I don't really know what ActiveP was trying to say

The truth is if you buy BTC from anyone, it comes from a miner. It doesn't matter who provides the coins, he had to run a miner to get them. You can actually trace the coins back if you're interested in their origin. If you're buying coins through a wallet or some mobile app, it's usually linked to an exchange. So either you're buying from an exchange without having to register there or the owner of the service is selling you his coins, that he bought on an exchange or mined himself.