gTLDs and ccTLDs often block certain names from ever being registered. In the case of gTLDs this includes the spec5 names as dictated by ICANN. For ccTLDs this entirely depends on the registry operator.
Which ever the reason for blocking names under a TLD, Centralnic offers registrars a downloadable list of domains that are unavailable for registrarion.
To find this list, log in to your registrar console
Locate the "FTP file browser" link under the "Graphs & Reports" section
Click on the LORDN folder
Here you fill find a downloadable list per TLD :)
Happy registrations !