The number of black elected officials at all levels of government rose by 356 during 1984 and reached 6,056 by the start of this year, with 85 percent of the increase coming from the South, according to the Joint Center for Political Studies.

Despite the gains, the Center said that "blacks still hold only 1.2 percent of the 490,800 elective offices in the country. Black elected officials continue to be concentrated in black majority districts and generally depend on the black electorate for victory."

The Center, a research group dealing with black issues, said "significant firsts in 1984" included the election of Ronald A. Blackwood in Mount Vernon, N.Y., as the state's first black mayor, election of Roxanne Jones of Philadelphia as Pennsylvania's first black woman state senator; election of Yvonne Miller of Norfolk as the first black woman since Reconstruction to be elected to the Virginia state House of Delegates; election of Margaret Carter of Portland, Ore., as the first black woman in the state legislature, and election of Jo Celeste Pettway as Alabama's first black female judge.

A setback for blacks, the report said, was the defeat of Rep. Katie Hall (D-Ind.) in the May 1984 House primary, which reduced the number of blacks in the House from 21 to 20. There are no black senators.

Overall, about 22 percent of the black elected officials are women, the report said.

Taking all offices into account, the biggest gains for blacks came in Alabama, where election of 61 new black officials raised the total to 375. In South Carolina, election of 47 new black officials raised the state total to 310. In Georgia, 39 new blacks were elected for a total of 340; in California, 38, for a 296 total; in Louisiana, 37 for a 475 total; in Florida, 36 for a 167 total, and in Texas, 32 for a 260 total.

Louisiana with 475 had the most black elected officials this January. It was followed by Mississippi with 444, Alabama with 375, Illinois with 357 and Georgia with 340.

Of the 6,056 black elected officials:

* By far the largest single group were members of city councils or similar city governing bodies. They totaled 2,189.

* The 1,368 black elected school board members were the second largest group.

* County board members were the third largest block at 534.

* In all, there were 20 black members of Congress, 90 black elected state senators and 302 black elected state representatives.

* There were 286 black elected mayors.

* There were eight black elected judges in the highest state courts and 359 judges in other courts.

* There were also 230 elected magistrates, justices of the peace and constables; 37 elected police chiefs, sheriffs and marshals; and 26 lesser elected judicial officials.