A private key blob (PRIVATEKEYBLOB) contains one complete public/private key pair. These key blobs are used by administrative programs to distribute 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 laptop computer). These key blobs can also be used by advanced applications where the key pairs are stored within the application, 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 Microsoft Base Cryptographic Service Provider format of private key blobs is fully documented in The Base Provider Private-Key Blob.