Blockchain to store private data and ownership As far as I know Bitcoin Blockchain can be accessed via API to enter data.
In this case I want to generate a hash/ Checksum MD5, that cannot be understood by anyone that doesn't have the private key, and store it blockchain. 

Give to the client the ID of the blockchain's record where hash is stored and the private key of the hash, would be ok, to ensure the ownership, immutability, and also the privacy of the stored data (a hashed .pdf file mostly)?

Thanks!
I think these links should be more useful for you than direct answer


http://www.cryptograffiti.info/ 
https://app.originstamp.org/home
https://bitcointalk.org/index.php?topic=1023190.0
https://github.com/petertodd/python-bitcoinlib/blob/master/examples/publish-text.py
http://www.righto.com/2014/02/ascii-bernanke-wikileaks-photographs.html