log blacklisted domains to a separate log file

[itpp2012]

1.7.7 is stable enough, even the current 1.7.8 is good enough.

Why not try it out on a test system and through some ab tests against it to
see what the performance does.

Or go for Lua to change the logfile, or create a separate server/location
block for the blacklist.

Context for access_log: 	http, server, location

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,254890,254894#msg-254894