Each spam filter has different criteria for being removed from the blacklist. You need to contact the source from which you're getting an indication that your mail is being blocked. Most reputable spam filters will list a web address you can go to in order to request removal from the blacklist.
A good site with information on how to implement blacklist can be found at: http://www.cauce.org/ http://relayblacklist.blogspot.com/ http://www.spamhaus.com/