If I ask you to pass on a message from me to someone else, and the message is encrypted using a symmetric key known to me, and to the recipient, but not to you, there's no security-by-obscurity in place.

If I ask you to pass on a message to someone, and the message is encrypted using a symmetric key known only to me, and I expect the recipient to be able to decrypt it because he knows that the key is written on the back of the paper, that's security by obscurity. You're just as able to do the decryption as the intended recipient is; I'm just hoping that you won't realize that.