An Address block is an entry in the clients keys.dat file. It looks like this:
[Bitmessage address in format BM-...] label = Name of Address shown in client enabled = true/false decoy = true/false chan = true/false privsigningkey = Hex representation of the key to sign message privencryptionkey = Hex representation of the key to encrypt message lastpubkeysendtime = Unix timestamp of last key broadcast noncetrialsperbyte = Integer showing difficulty payloadlengthextrabytes = Integer showing extra bytes required
Some values may not be present, if the Address was generated with earlier versions of the client.
Editing an Address Block can be dangerous. Users are advised to back up the keys.dat before doing so.
Deleting the address block removes the address from the client. More Information, when or why this should be done can be found in the plausible deniability article.