This is done by logging in to http://cp.blacknight.com and going to:
Account (or Home) >> Registered Domains >> domain name >> Manage Nameservers
When changing nameservers, you need to allow for up to 24 hours for full DNS propagation.
Please note: This is *NOT* done under: Hosted Domains, Websites, or Manage DNS. In the above you can only set the domain to use "External Nameservers" to remove the domain from our nameservers. You can not specify what nameservers you want the domain to use here.
If this is a domain transfer please ensure you press the "Manage Nameservers" to ensure you are viewing the current nameservers as the system can be slow to refresh