Macports installer public signing key The MacPorts installation guide says:


  
  Download the latest MacPorts-2.3.1-....pkg installer from the MacPorts download directory. [...]
  Double-click the downloaded package installer to perform the default “easy” install.
  


In between steps 1 and 2, however, a user may wish to verify the integrity of the downloaded package, using the PGP signature file also found in the Macports download directory.

If SIGNATURE_FILE is the name of the signature file, I get the following result:

$ gpg --verify SIGNATURE_FILE
gpg: Signature made Thu 26 Jun 00:37:12 2014 BST using DSA key ID B4AAE6CD
gpg: Can't check signature: public key not found


Where can I find the public key? Google doesn't seem to know.
The key ID is B4AAE6CD, from your command output. You need to search and retrieve the key from a keyserver: 

gpg --recv-keys B4AAE6CD


Then your verify command should work.