First -
DO NOT USE blockchain.info, they are untrustworthy and can steal your coins.
Webwallets are incredibly insecure. I recommend using Bitcoin Core or Electrum. Don't forget to encrypt your wallet with a strong password and to make backups!
Also, never, and I mean it -
NEVER copy-paste your private keys! Write them down on a piece on paper instead.
You should also make a Linux (such as Ubuntu or Debian) live USB (even if you already have an installation - it may be infected!) and boot from it to do the following.
Your casascius coin should contain a so-called mini private key under the shiny holographic label, it's going to look like S6c56bnXQiBjk9mqSYE7ykVQ7NzrRy.
This is a mini private key, which can be converted to a normal WIF one in multiple ways.
One simple way is to use bitaddress.org (Wallet Details tab) and entering your mini private key there.
WARNING: Do not use it online. I recommend downloading the repo offline (
https://github.com/pointbiz/bitaddress.org/archive/v3.3.0.zip) (download link for the zip linked at the bottom of bitaddress.org).
After downloading the zip, unplug your Ethernet cable, and/or disconnect from the WiFi (make sure to disable it!).
Now, extract it in your home directory (you should be using Linux to follow the tutorial, didn't I tell you?), open it in a file explorer and double-click on the file named index.html)
Congratulations - you're running bitaddress.org offline.
Now open the Wallet Details tab and enter your mini private key there. Your address along with the WIF private key will be displayed, carefully write it down and double check.
You can then sweep this private key in your wallet of choice.
Tip: you don't need to do that if you are using Electrum, as it already supports mini private keys.