Although I don't use NTL, I have received the download error message "rejected by tracker - Port 6882 is blacklisted." The site had this help: Why is my port number reported as "---"? (And why should I care?)
The tracker has determined that you are firewalled or NATed and cannot accept incoming connections.
This means that other peers in the swarm will be unable to connect to you, only you to them. Even worse, if two peers are both in this state they will not be able to connect at all. This has obviously a detrimental effect on the overall speed.
The way to solve the problem involves opening the ports used for incoming connections (the same range you defined in your client) on the firewall and/or configuring your NAT server to use a basic form of NAT for that range instead of NAPT (the actual process differs widely between different router models. Check your router documentation and/or support forum. You will also find lots of information on the subject at PortForward
http://portforward.com/).