The conservative political consulting firm of Black, Manafort and Stone has been fired by Paul S. Trible Jr., Virginia's Republican nominee for the U.S. Senate, one of the firm's partners said yesterday.

Paul Manafort, whose Alexandria-based company was hired on a $4,000-a-month contract by Trible last December, said Judy Peachee, the campaign manager, told him the dismissal was for budgetary reasons.

"But I can't believe that is the reason," said Manafort. "I think it was internal campaign politics." Manafort said he was surprised and disappointed by the decision, which came only days after Trible, a 35-year-old congressman from Newport News, was nominated by a GOP convention in Richmond.

Some conservatives viewed the dismissal of Manafort's firm as a sign of Trible's uneasy relationship with the conservative right in the Virginia Republican Party.

Campaign spokesman Neil Cotiaux said it was necessary to fire Black, Manafort and Stone to make room for Robert Weed, formerly with the Republican National Committee, who managed Trible's first successful congressional campaign, in 1976. "It makes no sense to have two people as talented as Weed and Manafort on the same campaign staff," said Cotiaux.

Black, Manafort and Stone has been identified both in Virginia and nationally with conservative campaigns. The firm handled President Reagan's campaign in 11 southern states in 1980 and in 1981 handled the campaign of former Del. Guy O. Farley for the Republican lieutenant governor nomination.

Manafort said yesterday the Reagan campaign had "elevated" Trible as Reagan's Virginia cochairman so he could get more "exposure" for this year's Senate race. "It is disappointing to have taken him this far and not take him through to the end," said Manafort.