A public key blob (PUBLICKEYBLOB) contains the public key portion of a public/private key pair. Unlike simple key blobs, these are not encrypted.
When communicating with someone who is not using the CryptoAPI, your application may need to build one of these key blobs manually so that the other user's public keys can be imported into your CSP. In addition, it is not uncommon for an application to display the value of a public key in plaintext form so the user can validate it by hand.
The Microsoft Base Cryptographic Service Provider format of public key blobs is fully documented in The Base Provider Public-Key Blob.