Harper was responding to reports about a group of lawyers seeking to cast doubt on Kenney's original claim that he exerted no influence over the ex-media mogul's application to be allowed to return to the country of his birth.
"Minister Kenney took every step to ensure that this matter was handled independently by public servants," Harper said during a news conference in Gimli, Man.
"It is not in the government's interest to intervene in this matter in any way, shape or form."
Kenney's office filed a grievance with the Law Society of Upper Canada against Guidy Mamann, a Toronto lawyer who said he found it improbable that the minister hadn't been involved in green-lighting the decision to let Black return.
The Law Society found insufficient evidence to investigate Mamann.
An open letter released Thursday, signed by some 80 lawyers, accused Kenney of trying to intimidate their colleague. They said they agreed with Mamann's assertion and dared the minister to complain to the Law Society.
"The use by an official of your office, of the Law Society of Upper Canada complaint process, in order to try to silence a critic for his opinion was rightly rejected by the Law Society," the lawyers wrote.
"However, if you believe that our statement violates the Law Society of Upper Canada Rules please feel free to report us to the Law Society. We find the attempt by you and your officials to muzzle freedom of expression to be reprehensible. We will not succumb."
Harper dismissed the letter.
"Minister Kenney, as you know, has filed a complaint with the Law Society in regard to the actions of a couple of lawyers, and so I think that complaint will need to be dealt with," he said.
"It obviously makes some spurious allegations against the minister."
Kenney's office also brushed off the letter.
"This (Black) decision was made by independent public servants, based on Canadian law," spokeswoman Ana Curic said in an email. "Neither the minister of immigration nor his staff were involved in processing this file."
The controversy revolves around a one-year temporary resident permit granted Black by the Citizenship and Immigration department.
Born in Montreal, Black gave up his Canadian citizenship in 2001 after being offered a peerage in Britain's House of Lords — something then-prime minister Jean Chretien had forbidden him from accepting while he held a Canadian passport.
Black — whose one-time Hollinger International media empire included London's Daily Telegraph, the Chicago Sun Times, the Jerusalem Post and the National Post, among others — was released from a U.S. prison earlier this year after serving a reduced sentence for fraud.
Black's controversial business dealings while at Hollinger's helm netted him fraud and obstruction of justice convictions in 2007, resulting in several years spent in prison in Florida before his release earlier this year.
— By Steve Rennie in Ottawa