Richard J. Davis' bid for a U.S. Senate seat from Virginia may have been damaged by a slight decline in Democratic strength among black voters, according to a survey of predominantly black precincts.

Davis outpolled his Republican opponent, Paul S. Trible Jr., by overwhelming margins in largely black neighborhoods, according to the Washington Post survey of unofficial returns from nine heavily black precincts in five cities.

Nevertheless, support for Davis in these precincts was several percentage points less than that given to Democratic Gov. Charles S. Robb in the same precincts in his 1981 victory over Republican J. Marshall Coleman. The Robb-Coleman race provides a possible benchmark for gauging Davis' backing among black voters.

Racial issues have weighed heavily in Virginia politics. According to census data, blacks account for 17.3 percent of the state's voting-age population. Despite his conservative political views, Trible courted black voters in an attempt to cut into traditional Democratic strongholds.

The survey indicated that Davis' support among black voters was slightly less than that for Robb in 1981 in precincts in Richmond, Norfolk, Newport News and the Southside city of Martinsville. Blacks account for more than 90 percent of the population in these precincts.

In two overwhelmingly black Richmond precincts, Davis got 95 percent of the vote to 5 percent for Trible. However, Robb received 98 percent in one precinct and nearly 97 percent in the other last year. In two Norfolk precincts, Davis led by margins of 94 percent and 93 percent. Robb had won 97 percent in both precincts.

In two Newport News precincts, Davis drew 94 percent and 92 percent. But Robb had scored higher, with 98 percent in one and 99.6 percent in the other. In one Martinsville precinct, Davis got 95.7 percent as compared with 96.4 percent last year for Robb.

In most of these precincts, the percentage of registered voters who went to the polls decreased slightly or held about the same as in 1981.