About confidential transactions blinding key
The blinding key is a shared secret among the receiver and the sender.

Typically the Diffie-Hellman key exchange is intiated by the sender ECDH, given that he already has knowledge of the receivers public key in a secure manner. The blinding key itself is not public knowledge, but the sender includes a one-time/temporary/ephemeral public key along with the transaction. The temporary public key is used in the ECDH key exchange scheme (receiver public key + temporary public key -> shared secret) to create the blind key.

There is no need for a private channel, the temporary public key can be public knowledge. The receiver however has to generate the shared secret themselves.