The new inquiry follows a review of a complaint made last November that officers has supplied information to those compiling such lists.
In a statement to Channel 4 News, the Met said: “In November 2012 the directorate of professional standards (DPS) received an allegation of an alleged breach of the data protection act by police.
“Due to the nature of the complaint it was initially non-recorded, however this decision was reviewed and amended by the DPS following an appeal by the complainant.”
Campaigners say that information in the files could only have come from undercover police officers spying on demonstrations.
The files resulted in people being denied employment after raising legitimate health and safety concerns or exercising their human right to belong to a trade union.
Last month in parliament, blacklisting was described as a “secret, insidious, shameful practice” and MPs heard allegations that the police and security services had supplied information.