Underwood was born in Tacoma, Washington, the son of Marilyn, an interior decorator, and Frank Underwood, a United States Army colonel. As a result of his father's military career, Underwood lived on bases in the United States and Germany during his childhood. Blair attended Petersburg High School in Petersburg, Virginia. He went on to attend the Carnegie Mellon School of Drama in Pittsburgh, Pennsylvania, and is an honorary member of Phi Beta Sigma Fraternity, Inc.

Underwood's 1985 appearance on The Cosby Show landed him a short stint on the ABC soap opera One Life to Live, which eventually led to his Golden Globe-nominated performance on the TV series L.A. Law. Underwood broke into film with roles in Set It Off, Krush Groove, Just Cause, and Deep Impact. He also had a supporting role as a geneticist in Gattaca. In 2000, he played the lead role in the short-lived television series City of Angels. In 2003, he guest starred in four episodes on the popular HBO series Sex and the City playing Cynthia Nixon's love interest. In 2004, he played the role of Roger De Souza opposite Heather Locklear in NBC's LAX. He also has appeared in a guest role as a grade-school teacher on the CBS sitcom The New Adventures of Old Christine. In 2007, he guest starred in an episode of the NBC series Law & Order: Special Victims Unit. His latest role is as the recurring character Alex in the HBO series In Treatment.

Underwood has received four NAACP Image Awards for such roles as in the docudramas Murder in Mississippi and Heat Wave.

Underwood was voted one of People's "50 Most Beautiful People" in 2000, and one of TV Guide's "Most Influential Faces of the 90s".

In 2007, Underwood co-authored the fiction novel Casanegra: A Tennyson Hardwick Novel with husband-and-wife team Steven Barnes and Tananarive Due.

--Wikipedia