The Supreme Court ruled unanimously yesterday that state financial aid to a blind student who was studying to be a minister does not necessarily violate constitutional requirements for separation of church and state.

Justice Thurgood Marshall, writing for the court, said the state of Washington's program of vocational aid for the blind was not intended to promote religion and it did not do so, since "no more than a minuscule amount of the aid awarded under the program is likely to flow to religious education."

The Washington Supreme Court ruled that the First Amendment barred the state from giving aid to Larry Witters, who was legally blind and studying at a Christian college to become a pastor.

Marshall reversed that ruling, but did so in a way that makes it uncertain whether Witters will receive any assistance.

Marshall, repeatedly emphasizing that the ruling was narrow and based "on the facts as they appear in the record before us," sent the case back to the Supreme Court of Washington for further review.

The state court based its ruling on the U.S. Bill of Rights. Marshall noted that the Washington Supreme Court could rule that Witters could not be given the aid under the "far stricter dictates" on separation of church and state in the Washington constitution.

In addition, Marshall declined to discuss the state's argument that giving Witters the assistance would "entangle" the state in religious matters in a manner prohibited by the court's past rulings. The state court, Marshall suggested, "may also choose to reopen the factual record" to consider those arguments.

Marshall said "as far as the record shows, vocational assistance provided under the Washington program is paid directly to the student, who transmits it to the educational institution of his or her choice. Any aid . . . that ultimately flows to religious institutions does so only as a result of the genuinely independent and private choices of aid recipients."

Marshall said the fact that the aid in this case went directly to individuals distinguished it from federal and state parochial aid programs struck down by the court last year.

Despite the narrow holding in Witters v. Washington Dept. of Services for the Blind, William J. Bennett, secretary of education, said the ruling was "heartening" because it "recognized that as long as government assistance is provided to students on a neutral basis, students may use such aid to attend the schools of their choice . . . . "

Bennett said that reasoning would sanction the administration's proposal for educational vouchers.

In other action yesterday, the court:

*Granted a request by the Reagan administration in Dole v. Railway Labor Executives' Association to let it test railroad train crews for drugs or alcohol after a serious rail accident. The regulations, which had been temporarily halted by an appeals court, also would allow supervisors to require breath and urine tests whenever they believe any crew member might be under the influence of drugs or alcohol.

*Let stand rulings that dismissed a lawsuit by the widow of Bruno Hauptmann, executed 50 years ago for the kidnaping and murder of Charles Lindbergh's son. Anna Hauptmann, 86, sued federal and state officials, saying her husband was innocent and the victim of a conspiracy. The case was Anna Hauptmann v. David T. Wilentz.

*Let stand a $1 libel award to Nobel Prize winner William B. Schockley, who said he was libeled by a 1980 story in the Atlanta Constitution about his controversial theories of heredity and intelligence. Shockley argued the jury award should be set aside for a new trial. Shockley v. Cox Enterprises.