In an effort to promote its controversial education reform programme, No Child Left Behind, the education department paid Armstrong Williams "to regularly comment on NCLB during the course of his broadcasts" on a nationally syndicated television show.
According to documents obtained under the Freedom of Information Act, Mr Williams had to interview the former education secretary Rod Paige, who is also black, for television and radio spots.
Mr Williams, 45, described by the Washington Post as "one of the most recognisable conservative voices in America", also exploited his contacts with a professional group of black broadcasters, America's Black Forum, "to encourage producers to periodically address" NCLB, and persuaded the television personality Steve Harvey to twice invite Mr Paige on to his show.
Mr Williams said yesterday he could not recall whether he had informed his viewers and listeners of the contract and that he had "an obligation to be more vociferous about the fact" if he did it again.
"I can certainly understand why people would think it was unethical," he said, arguing that the money had paid for public service announcements to promote a policy that he had always believed in.
In public debates, Mr Williams describes himself as a pugnacious, provocative and principled voice for conservatives and Christian values.
Mr Williams, a former aide to the black conservative supreme court justice, Clarence Thomas, is a syndicated columnist whose work has appeared in the Los Angeles Times and Detroit Free Press, among others.
He has syndicated television and radio shows and he also makes regular guest appearances on political talkshows.
The contract was part of a $1m deal with the public relations company Ketchum to produce "video news releases" that looked like news reports - a device criticised by the government accountability office last year when used to push the administration's drug prescription plan.
Ketchum refused to comment.
The Bush administration heavily targeted black voters during the last election in an attempt to undermine the Democratic party's most loyal base.
Nationally, Mr Bush's share of the African-American vote rose from 9% to 11%. But in the key swing states of Florida and Ohio, his share almost doubled from 7% to 13% and 9% to 16% respectively.
The senior Democrat on the education committee, George Miller, told USA Today that the contract was "a very questionable use of taxpayers' money" that is "probably illegal".
Melanie Sloan, of Citizens for Responsibility and Ethics in Washington, said the contract could be illegal because Congress had "prohibited propaganda" or lobbying for government-funded programmes "and it's propaganda".
Bob Steele, a media ethics expert at the Poynter Institute for Media Studies, said: "I respect Mr Williams' statement that this is something he believes in. But I would suggest that his commitment to that belief is best exercised through his excellent progressional work rather than through contractual obligations with outsiders who are, quite clearly, trying to influence content."
A spokesman for the education department told USA Today that the contract followed standard government procedures, but that there were no plans for "similar outreach" work.
When asked whether he would do it again, Mr Williams said yesterday: "It's a judgment call".