I just enabled blacklisting option "-b 32000" on sshguard 1.5 as a workaround (which works) for this bug https://bitbucket.org/sshguard/sshguard/issues/19/blacklisting-is-on-by-default-and-cannot
Using "-b 32000" (without a filename) actually works, while the man page says
"The -b command line option enables blacklisting and requires the filename to use for permanent storage of the blacklist. Optionally, a custom blacklist threshold can be prefixed to this path, separated by ':'."
"requires the filename" is misleading if meant to be mandatory only for permanent storage. Should better say something like
The -b command line option enables blacklisting. Optional values are a blacklist threshold (default: 40) and/or a filename (filename is required for permanent storage of the blacklist). If both values are given, they need to be separated by ':'.
If both threshold and filename are optionally, the syntax should be something like