if it is 51 chars and starts with a 5... you have an "uncompressed private key in WIF" format.
You can double check if you have written it down correctly by using bitaddress.org.
DO NOT type your private key straight into the online version!! You need to download a copy of the bitaddress.org website (use the link at the bottom of the page for their Github or "zip"). Run it offline on your PC, then select "Wallet Details", and type your key into the box and click "view details".
You should see the matching "uncompressed public key". If you don't, then your private key is incorrect.
Also, to be honest, using blockchain.info wallet is a very bad idea. Their fee system is broken at the moment, and if you try to send your coins anywhere, chances are it will use a stupid low fee and your transaction will get stuck. I'd recommend getting Electrum and "sweeping" your key into a new "seeded" wallet.
Instead of doing the bitaddress thing, you can also try typing the key into Electrum "sweep" dialog, if the "Sweep" button stays greyed out, then the key is invalid and likely you have transcribed it incorrectly
Feel free to PM is you want more help.