Port is blacklisted

From AzureusWiki

Which port should I use for Azureus?

1. Choose a port from the 49152–65534 range. Azureus needs only one port, but you have to forward it for TCP and UDP to avoid NAT problems and to make sure DHT is running ok. If you really need to choose a port within the range 1024–49151 make sure you pick an "unassigned" port listed at the IANA port list.
2. Go to Azureus > Tools > Options > Connection and input the new TCP listening port that you have chosen.
3. Save the changed options.

Why ports are blacklisted and what to do about it

ISPs have been increasingly reducing the available bandwidth for P2P users on the standard file-sharing port ranges. You may find you need to keep changing your port on a regular basis. This is normal, and a port change should sort you out if this is indeed what is happening. And please keep in mind that after any port change, and stopping and restarting any torrent, it may take up to half an hour or more for the benefits to become noticeable. So please try not to panic when things do not improve immediately. This is normal. :)

So why is port 6881 (or whatever) blacklisted?

To avoid a decreasing Average Swarm Speed, some tracker administrators are banning these often throttled standard ports. This includes the standard port range of 6881–6999.

So what happens is, those trackers reject any IP addresses trying to connect who are listening on any of the ports within those ranges, and some of those trackers may blacklist those IPs for 48 hours. So the torrents show red faces, and even if you change your listening port, you are unable to connect because of the 48 hour ban. This kind of ban will often be reported under the "Tracker Status" of a torrent, with the blacklisted information.

So what can I do about it if I'm already blacklisted?

1. Choose a new TCP listening port for Azureus. The best range to choose a new port from is: 49152–65534 If you really need to choose a port within the range 1024–49151 make sure you pick an "unassigned" port listed at the IANA port list. Remember that Azureus needs one listening port only, so please choose one port from the above range.
2. Go to Azureus > Tools > Options > Connection and input the new TCP listening port that you have chosen.
3. Click "Save" at the bottom left side of the screen.
4. Go into your router and/or firewall, if any, and forward the port you have chosen using the methods as described in NAT problem, Port forwarding and Router configuration. Remember to forward that port not only for TCP but also for UDP protocol, DHT needs that! (If your router is using UPnP, it should update automatically.) The "Port xxxx is blacklisted" message from the tracker should disappear and the torrent will start. If not, check your router and firewall, then go on to step 5.
5. Stop all your actively running torrents. (This is temporary, just until you get the port sorted out. Azureus will remember all the peers you were connected to when you start the torrents up again, even if the tracker is offline. No need to panic. Please ring a friend if you need any extra support during this trying experience.) :-)
6. Quit Azureus.
7. Wait a few minutes.
8. Run Azureus
9. Go to Tools -> NAT/Firewall Test and do the port test. You will see a port tester with 6881 entered in it by default. Change 6881 to the port number you chose earlier and click "test". You will then get any one of three results from this:
   1. "This port can not be tested because it is already in use" If you get this message, repeat steps 4–8
   2. "NAT Error" If you get this message, please see the Port forwarding and/or the RouterConfiguration page.
   3. "OK!" If you get this message, then you have just done everything right, all is groovy, and the world should appear a much rosier place to live in. :-)

Read the Azureus FAQ