For the first time in its nine years in the radio broadcasting business, the Mutual Black Network is now wholly owned by black investors.

In an agreement concluded in recent days, Sheridan Broadcasting Corp. of Pittsburgh acquired for $1.2 million the remaining 51 percent in Mutual Black Network that it did not own previously.

Sheridan, owned by a group of black investors in the Pittsburgh area, bought a 49 percent share of the network from the Mutual Broadcasting System in 1976. MBS, now based in the Washington area, started the black programming network in 1972.

According to Thomas McKinney, the president of Sheridan, Mutual Black Network now has 91 affiliated radio stations around the country. In the Washington region, affiliates include WUST in the District; WEBB in Baltimore WILA, Danville; WPCE and WOWI (fm) in Norfolk-Portsmouth; WENZ, Richmond and WTOY, Roanoke.

McKinney said yesterday that the network is planning a name change in the near future-- possibly to Sheridan Broadcasting Network. The company also owns four other radio stations -- AM and FM outlets in Pittsburgh and stations in Boston and Buffalo.

The Mutual Black Network offers a variety of news, sports and information programs, including hourly newscasts during the day. Under an agreement with Mutual Broadcasting, which now is owned by Amway Corp., Sheridan will continue to share broadcast facilities at Mutual's headquarters in Arlington.

Sheridan has managed the network since 1976 but could not gain total ownership until government rulings were made final, permitting the black programming network to sublease broadcast lines from Mutual. Mutual in turn leased lines from American Telephone & Telegraph Co. The black network could not afford to lease lines separately from AT&T.

Since 1976, revenues of Mutual Black Network have grown by about 300 percent to a projected $3 million in the current fiscal year, McKinney said. We started from a very small base . . . I see two or three years ahead of that type of growth, before expansion levels off at about 20 percent a year, he added.

Most radio stations affiliated with the network now cater almost exclusively to black audiences, but McKinney said this is partly because of the network name. With the name change, McKinney said, he hopes to attract a number of information-oriented stations that often affiliate with more than one network.

Mutual Black now offers regular medical, finance, commentary and environmental programs but, because of costs, popular broadcasts of black college football games have been dropped this season.

Mutual is seeking government approval, however, for its own system of satellites to transmit its programming, and if the Federal Communications Commission approves, the balck network would lease a full line of its own and be able to expand programming significantly, McKinney said.

We want to become more involved in Third World activities, particularly in Africa and the Caribbean . . . we have plans for an hour-long, daily public affairs magazine-type program . . . some stations are interested in drama, he said. Live sports events coverage also could be resumed with the proposed transmission system.

Mutual Black Network has about 50 employes, most of them based in Washington. McKinney previously directed the network from offices here but moved to Pittsburgh to head Sheridan.