Taft Broadcasting Co. has purchased an equity interest in Black Entertainment Television, a District-based cable television programming service, the two companies announced yesterday. The purchase terms were not disclosed.

Taft, based in Cincinnati, owns Channel 20 here, six other television stations, 12 radio stations, other programming services, amusement parks--including King's Dominion--and a 47 percent interest in the Philadelphia Phillies.

Denver-based Tele-Communications Inc., the nation's third largest cable system operator, has been a partner in BET since it began operations in 1979. TCI is also involved in cable partnerships with Taft and, by next month, that venture will own systems with over 120,000 subscribers that will be worth, according to Taft, about $100 million.

"It's the financial and programming support we've been looking for," BET President Robert L. Johnson said in an interview yesterday. Johnson, who has also been seeking a cable franchise for the District, said the Taft transaction will not be involved in that separate venture.

"This will enable us to become a major black television network at a time when the major urban markets are being wired for cable," Johnson said. Hanna Barbera Productions Inc., which produces animated programming, Ruby-Spears Enterprises Inc., Worldvision Enterprises Inc., another program distributor, are Taft subsidiaries.

For the nine-month period ending Dec. 31, 1981, Taft reported sales of $307 million and profits of $36 million, both gains of about 50 percent over 1980 results.

Currently, BET's sports, films and other programming is shown Friday evenings from 11 p.m. to 2 a.m. on the USA Network, another satellite network that reaches 9.6 million subscribers. But the firm announced plans recently to expand its satellite transmitted schedule to seven days a week, 8 p.m. to 2 a.m.

"We are committed to Black Entertainment Television's long range programming objectives, and we believe that BET's business concept of providing 42 hours a week of specialized programming for the black community is a venture that is both economically sound and socially rewarding," said Charles Mechem Jr., Taft's chairman.

Johnson said the funding that Taft would provide would permit BET to expand its operations and buy more independent programming. He said he is negotiating with producers about purchasing programming.

Since its inception, BET is essentially dependent on sports and old films, but the network's expanded programming format is set to also include public affairs shows, a talk show, music and other programming aimed primarily at black audiences