''It looked like a blacklist, it smelled like a blacklist,'' says Barry Lynn, legislative counsel for the American Civil Liberties Union. ''If any letter has a chilling effect, this one froze people.'' The document in question is a letter that Attorney General Edwin Meese's Commission on Pornography mailed to 23 retailers last February saying that the panel had ''received testimony alleging that your company is involved in the sale or distribution of pornography.'' The letter also contained an ominous invitation: ''The Commission has determined that it would be appropriate to allow your company an opportunity to respond to the allegations prior to drafting its final report section on identified distributors.'' Failure to answer the charges within three weeks, said the commission, would ''be accepted as an indication of no objection.'' To many companies, the letter was a veiled threat of public censure and even criminal prosecution against companies that sell ''adult'' publications. At least six retail chains that received the letter, with 8,632 outlets nationwide--most notably the Southland Corp., owner of 7-Eleven convenience stores--have stopped selling Playboy and Penthouse. Playboy, along with the Magazine Publishers Association and other groups, is suing the commission to retract the letter and issue a statement explaining its intentions. They charged that the letter has touched off a ''blaze of censorship across the land.'' Judge John Garret Penn is expected to rule on the case before the scheduled release of the commission's controversial report on pornography July 3. The commission said it had ''inadvertently'' dropped from its letter the name of the man who had supplied the list of suspect companies: the Rev. Donald Wildmon of Tupelo, Miss., a Methodist minister and founder of the ultraconservative National Federation for Decency. The Justice Department vigorously denied that it plans prosecutions and insisted that its ''fair-reply letter'' was not a McCarthyite tactic. ''There is not a blacklist,'' says Government Attorney Robert Cynkar. ''There never was.'' Playboy's lawyer, Bruce Ennis, asserts that the commission's letter was drafted specifically to intimidate the magazine ''because it couldn't do so using the law.'' No jury has ever found Playboy to be legally obscene under the guidelines prescribed by the Supreme Court, although individual issues of its rival Penthouse have been ruled obscene. Meanwhile, in Maine, the drive against pornography ran into a political roadblock. Conservative activists had proposed a referendum that would have made the selling or promotion of obscene material a criminal offense. In the highly charged debate that raged throughout the state before voting last Tuesday, supporters of the proposal argued that pornography was leading to rape and child abuse; opponents claimed that the legislation would result in widespread censorship and perhaps even book burning. With 38% of Maine's registered voters casting ballots--an uncommonly high turnout for a Maine primary--the referendum was defeated by a more than 2-to-1 vote.