A private key blob (PRIVATEKEYBLOB) contains one complete public/private key pair. These key blobs are used by administrative programs to distribute and/or transport public/private key pairs; for example between a network administrator's computer and a user's computer or between the user's desktop computer and his or her laptop computer. These key blobs can also be used by advanced applications that want to store key pairs themselves, rather than relying on the CSP's storage mechanism.
As the private key portion of the key pair is extremely confidential, these blobs are typically kept encrypted with a symmetric cipher.
The format of private key blobs is fully documented in the Private Key Blob Format section.