The University of Virginia, whose application deadline for the fall term was Feb. 1, has announced it will accept applications from black students until shortly before classes begin in an effort to meet the goals for black enrollment set in a desegregation plan negotiated with the federal government.

George Mason University and the College of William and Mary, which come under the same desegregation plan, also have notified high school guidance counselors around Virginia that they are extending indefinitely their application deadline for blacks.

Officials said all students admitted would be qualified to do college work, though they expected that, as in the past, the average admissions test scores and high school grades of blacks would be considerably below those of whites. All three state-supported schools will provide special summer classes and term-time tutorials primarily for blacks whose records indicate they need the help.

Last month a federal judge threatened to cut off federal funds to Virginia and other states if they failed to make "substantial progress" by next March in enrolling more blacks in predominantly white state colleges.

"We're going to do everything we can to get more qualified black students," said Jean Rayburn, dean of admissions at the University of Virginia in Charlottesville.

Last fall, when it raised admissions standards, the proportion of blacks in the university's freshman class fell from 13 to 10 percent, the first major drop since the school began a substantial desegregation effort a decade ago.

For next fall's class, Rayburn said, Virginia received a record 12,900 applications by its Feb. 1 deadline, and accepted the smallest overall proportion ever, just 32 percent. The number of black applicants fell by about 14 percent, she said, partly because "the word got out that we were more selective and some students were discouraged."

Rayburn said applications from blacks are considered separately and initially are reviewed by black admissions officers. So far, she said, the acceptance rate for blacks has been 63 percent--about double that for whites.

Rayburn said the university could not follow a color-blind admissions policy because the average high school grades and Scholastic Aptitude Test (SAT) scores of blacks are lower than whites. According to national figures, the difference in SATs is about 100 points. "Very honestly, we want to have a substantial number of black students," she said. "We have to make some adjustments."

Under the state's college desegregation plan, accepted in late January by the office of civil rights in the U.S. Department of Education, the number of black Virginia residents enrolled in the university as new freshmen or transfer students is supposed to rise next fall by about a third to 152. By 1985, the figure is expected to reach 269, which is the university's share of an effort to make the proportion of black Virginia residents attending the state's 13 predominantly white state colleges almost match the proportion of black high school graduates in the state.

Although black enrollments increased under an earlier desegregation plan adopted in 1978, the plan failed by a wide margin to reach its goals. Similar plans also have fallen short in other southern states. This prompted the action by the NAACP Legal Defense Fund, which resulted in last month's order by U.S. District Court Judge John H. Pratt, requiring the federal government to enforce specific deadlines.

Virginia Gov. Charles S. Robb has criticized Pratt's timetable for a court review next March, but state Education Secretary John Casteen III, a former admissions dean in Charlottesville, has asked the state universities to make major efforts to meet it.

College officials said they decided to extend their application deadlines for blacks after receiving letters from Casteen, although he did not specifically request such action.

Blacks make up only 3 percent of undergraduate enrollment at George Mason in Fairfax County and about 5 percent at William and Mary in Williamsburg. Under the desegregation plan, the number of black freshmen and transfer students is expected to more than double at George Mason next fall and to rise by about two-thirds at William and Mary.

Officials at all three colleges said the extended deadlines would have no impact on whites already admitted to the schools. But they said the number of whites accepted later from waiting lists would probably be cut as more blacks are enrolled.

Nationally, the average Scholastic Aptitude Test scores of blacks are about 100 points below that of whites in both the math and verbal parts of the exam. Rayburn said about the same pattern prevails among black and white students at the University of Virginia. According to the most recent university figures, about 77 percent of white freshmen graduate within five years, compared to about 58 percent of blacks.

At William and Mary, admissions dean Gary Ripple said black freshmen last fall averaged 83 points below the college-wide average of 587 in the SAT verbal test and 93 points below the 613 college-wide average in SAT math.But Ripple stressed that the scores for blacks still were considerably above nationwide averages and that all those admitted have "a good chance" to complete their college work.

"It's a fact of life . . . that a judge has ordered that we must make every possible effort to end de facto segregation in Virginia," said Ripple. "I suppose it sounds like we're doing this because we are forced to. But we also believe our college will benefit from having a greater representation of blacks."