he Georgia Supreme Court agreed yesterday to review a trial court's decision to take a 3-year-old white boy away from his white mother after she gave birth to an illegitimate, racially mixed daughter.

Kathy Blackburn, 26, is seeking to regain custody of her son, Nicholas, from her former mother-in-law, Nancy Blackburn. Both women live in Millen, an East Georgia town of 3,700.

The custody dispute has drawn a number of "friend of the court" briefs from groups concerned with civil liberties, abortion and racial discrimination, including one from the Georgia Right to Life Committee Inc., which said that, if left to stand, the decision would encourage abortions.

Kathy Blackburn was given custody of her son after her divorce from Mark E. Blackburn on July 17, 1979. Nearly two years later, on May 13, 1981, she gave birth to a second child, a racially mixed daughter named Jennifer.

After hearings, Superior Court Judge W. C. Hawkins gave permanent custody of the boy to his grandmother, citing "clear and convincing evidence" that Kathy Blackburn had "failed to provide adequate supervision, moral guidance and medical attention as are necessary for the child's well-being" and "has given birth to an illegitimate female child."

Although Hawkins' order did not mention race, the appeal and supporting "friend of the court" briefs filed with the state Supreme Court claimed the decision was discriminatory because it made no finding as to Kathy Blackburn's unfitness as a mother to Jennifer.