Sen. Harry F. Byrd Jr. (I-Va.), who has been criticized for not supporting a woman or black for one of the four new federal judgeships in his state, has given his blessing to a woman as a possible nominee for U.S. attorney in eastern Virginia.

In a letter released this week Byrd said that while he continues to follow a practice of not making recommendations for federal prosecutors, "I am willing to say, without foreclosing others, that I would support in the confirmation process either Mrs. Susan G. Moenssens or Anthony F. Troy."

The Justice Department, which received Byrd's letter, is seeking a replacement for Republican William B. Cummings, who announced last month that he will resign effective June 1.

A Justice spokesman said yesterday that "many names have been accumulated" for the position, and that Moenssens and Troy are among a large group under consideration.

Moenssens, who will be 38 tomorrow, has a private law practice in Richmond with her husband. She is a graduate of the T.C. Williams School of Law at the University of Richmond.

Troy, 37, served as Virginia attorney general for a year after Andrew P. Miller resigned to run unsuccessfully for the Democratic nomination for governor in 1976.

In his April 2 letter to Associate Attorney General Michael J. Egan, Byrd said, "I am aware . . . of President Carter's desire to appoint women to such positions and would be pleased to cooperate should Mrs. Moenssens he selected."

Byrd said "although not well known to me, Mrs. Moenssen's professional credentials have been highly commended to me by a attorneys in whom I have confidence, including the dean of the University of Richmond law school, the president of the Metropolitan Richmond Women's Bar Association and the president-elect of the Henrico County Bar Association."

In a telephone interview yesterday, Mrs. Moenssens said she had received "a lot of support from the male power structure."

She also said she was "overwhelmed by the strong support from my male colleagues in the legal profession and esteemed persons in political office . . .

"I want to put to rest recurring rumors that women are being discriminated against in Virginia in nominations for presidential appointments. As a professional woman, I can state with confidence that there is no sex discrimination today in appointments to public office."

Byrd, who is consulted on presidential appointments even though he quit the Democratic party several years ago, called Troy "an able attorney."

A spokesman for Byrd said Egan visited the office of Virginia's senior senator last Friday and showed him the names of a number of lawyers being considered for the appointment but the Byrd did not know others on the list.