The all-Democratic Montgomery County Council, intent on appointing a black independent to its influential planning board, discovered yesterday the post must be filled by a Republican.

Herrington J. Bryce, president of the D.C.-based National Policy Institute, had won endorsements from five of the seven council members and was about to become the first black appointed to the board.

Council President Neal Potter was in the middle of his endorsement when he learned that the state's Regional District Act, which governs appointments to the board, requires that unexpired terms must be filled by a member of the same party.

The council deferred the appointment until next week, pending study by county attorneys.

The blunder left council members embarrassed and may throw out a selection process that began in October.

The five-member planning board is the official advisory body to the council on all planning and zoning issues. The new appointee will replace Republican lawyer Sheldon Schuman, who resigned.

The appointment is sensitive since state law forbids each political party from holding more than three positions on the board. Three members are already Democratic. County Republicans, who endorsed retired builder Robert Brennan, charged that Bryce would dilute Republican influence.

"If the council feels it's a choice of appointing an independent or a Republican, they'll pick the independent," charged Barbara Mayle, vice chairman of the county Republican central committee. "It's blatantly political."

But Bryce appeared confident that he might still get the post. "I'm certain that they will find an honorable way out of this."