The number of black elected officials at all levels of government increased last year, yet blacks held only 1.3 percent of the nation's 490,000 elective offices, according to the Joint Center for Political Studies.

The center, a think tank on black issues here, said blacks held 6,424 offices at the end of 1985 compared with 6,056 at the beginning of that year, a 6.1 percent increase, with gains occurring in every region and every type of office.

Two important "firsts" in 1985, the center said, were the election of L. Douglas Wilder as lieutenant governor in Virginia, the first time since Reconstruction that a black won a major statewide office in the South, and the election of Alyce Clark to the Mississippi legislature, the first time a black woman has won such an office in that state.

Of the 6,424 elected blacks, 1,483 are women.

The report said Mississippi had the most black elected officials, 521, followed by Louisiana, 488; Illinois, 426; Georgia, 417; Alabama, 403; South Carolina, 329; Arkansas, 315; Michigan, 314; California, 287, and Texas, 281.

The report said there were 251 black elected officials in the District of Columbia (most of them members of advisory commissions), 108 in Maryland and 125 in Virginia. Idaho, Montana, New Hampshire and North Dakota had none.

The number of elected blacks in southern states corresponds to national population patterns. Over half of the nation's blacks live in the South.

Linda Williams, a political scientist at the center, said that while the number of blacks in elective office is growing, the rate is "dramatically slower" than before 1976, when the average increase was more than 18 percent.

She said before 1976 the number grew rapidly as blacks, freed of political restrictions by civil rights laws, won office in majority-black communities, such as small towns, where they had been excluded from the political process. Growth slowed in part because blacks tend to live in tightly concentrated areas and have relatively little voting strength outside those areas. Though there are still 300 small towns with black majorities and white mayors, she said, blacks must seek office in communities where they are not the majority if they are to expand their elected positions.

The figures show no black governors or U.S. senators but 20 U.S. House members.

At the state level, blacks hold four elected administrative jobs, 92 senate seats and 304 house seats.

At the county level, they hold only one elected executive seat, but 596 seats on governing bodies and 84 other official jobs.

In cities, there are 289 black mayors, 2,396 blacks on city councils and municipal governing boards, 236 on advisory commissions and 118 in other elected positions.

In law enforcement, blacks hold eight elected seats on the highest state courts, 366 other elective judgeships, 233 positions as magistrates, justices of the peace and constables, 28 other judicial offices and 41 elected sheriff, police chief and marshal jobs.

In education, there are nine blacks on state education agencies, 46 on university and college boards, 1,473 on local elected boards of education, and a dozen in other offices.