The Montgomery County Board of Education approved several changes early today designed to improve racial balance at Montgomery Blair High School in Silver Spring, which has the highest minority enrollment of any high school in the county.

The board's action substantially returns to an enrollment plan aimed at bringing a large number of white students into the school that the previous board majority, toppled in last November's elections, rejected more than a year ago. The State Board of Education subsequently overturned the old board's action, ruling that it did not sufficiently address the question of racial imbalance at Blair.

Last night's action was an effort to meet the concerns of the state board.

School officials said the changes will result in enrolling about 400 more students, most of them white, at the school by 1988. Those students had been assigned by the previous board to Einstein High School in Kensington.

In addition, the board voted to begin an accelerated computer and science program at the school by 1985. This plan is expected to add almost 300 more students, also mostly white.

Blair now has a minority enrollment of 61 percent. The average number of minorities in county schools is 25.4 percent. Last night's action is expected to drop the minority enrollment at Blair to about 50 percent by 1988, when the plan is expected to be fully implemented.

All six members present, including board member Marian Greenblatt, who last year opposed such an attendance plan, supported the new assignments.

"Unlike prior board decisions, this one is not a matter of cutting Blair off at the Beltway and isolating it," Board President Blair Ewing said. "By our actions tonight, we have tried to reduce racial imbalance and make Blair a viable place."

More than a year ago, when the previous board rejected the assignment of students to Blair who live north of the Capital Beltway, several board members said they decided to do so because parents in that area said they would withdraw their students from public school rather than send them to Blair.

The board also voted last night to reaffirm an earlier decision to close Peary High School in June 1984 and to close Northwood High in June 1985. Northwood previously was scheduled to close at the end of next year.

Once the board made those two decisions, which had deadlocked it twice last week, the board acted on the Blair area. School board members were concerned during their discussions last week that if Northwood remained open, the board then would have to make substantial boundary changes to reduce Blair's minority enrollment.

Under the plan adopted last night, students from Forest Knolls Elementary, Pine Crest Elementary and the former Four Corners Elementary area will feed into Blair.

When the previous board rejected this boundary change, the state board ruled that it reduced overall enrollment at the school by 400 students and had ignored a viable plan that could reduce the serious racial imbalance problem at Blair. In so doing, Blair's educational program, which must meet the needs of a diverse student body, was hampered, the state board ruled.

Members of the Blair community present last night welcomed the board's boundary changes, but questionned whether the accelerated so-called "magnet" program in computer science would draw as many white students as expected. They also worried whether delayed enrollment of students from the three elementary areas until 1985 might come too late to address what they consider a critical problem.

"I'm glad the board added these other students, but I'm concerned that it is not soon enough," said Barbara Cantor, president of the Blair Advisory Committee.

At the same time the board added three predominantly white areas to the Blair feeder pattern, a motion that would have removed New Hampshire Estates Elementary from the Blair area was rejected by the board. New Hampshire Estates has the highest minority enrollment of any school in the county at 83 percent. In rejecting a motion to remove the school from the Blair area and feed students into the Springbrook High area, board members said they would reconsider the school next year when a review of the Springbrook area is planned.