The exciting back - who can play wing or full back - has signed a long-term deal to move to BT Sport Cardiff Arms Park from Leicester Tigers ahead of the 2015/16 campaign.
Scully, 27, is an established international having made a total of 21 appearances for his country, scoring seven tries, and featured in the 2011 Rugby World Cup for the Eagles where he played in all four matches.
The Sacramento-born ace - who stands 6ft 4in and weighs 14st 9lb - joined Leicester Tigers on a trial basis in 2013 and earned an extended contract after his eye-catching performances.
Scully marked his Leicester debut with a try in a victory over Newcastle in 2013 and has made 34 appearances for the Tigers, scoring a total of six tries.
Billy Millard, Head of Elite Performance at Cardiff Blues, said: "Blaine is a fantastic acquisition and we're all excited about him joining us next season, following his expected involvement with the USA at the Rugby World Cup.
"Blaine is a determined, courageous and talented player who will add real attacking options to our back line. He is a powerful runner with a strong skill-set in the air, is strong defensively and knows how to score tries.
"I was lucky to work with Blaine during my time with USA Rugby and so know the tremendous attitude, enthusiasm and professionalism he will bring with him to Cardiff Blues. He is a class act on and off the field.
"Blaine drives high standards and has earned the respect of players and coaches at Leicester. He will buy into the ethos we are developing here and is determined to really add something to the squad we're building."
A history graduate at California University, Scully only started playing rugby at college but went on to become a four-time All American, captain of the 2011 USA Rugby Collegiate All-Americans touring squad and won two national titles.
Scully made his international debut for the USA in June 2011 and scored a memorable hat-trick of tries against Japan in the Pacific Nations Cup in 2014.
Scully said: "I am beyond excited for the opportunity to join Cardiff Blues and the new challenge.
"In speaking with Billy Millard, I found the ambitions of the region and the opportunity to develop very attractive. I will do my very best to contribute meaningfully to the team and have a positive impact on the culture. I am looking forward to taking the next step of my career there.
"I would like to thank the entire Leicester Tigers organisation: players, coaches, staff, and fans. I have had a wonderful two seasons there and will always have a special fondness for Welford Road.
"I will leave the club a better player, man and with mutual good will. I have made many lasting friendships and wish them all the best in the future.
"Currently, my focus is firmly on the conclusion of the Premiership season, where we hope to complete the campaign with a strong finish.
"Moving forward to the summer, the USA will have an intense competition schedule preparing for the World Cup in England. After a hopefully successful World Cup campaign, I will move to Arms Park to begin my first season as a Cardiff Blue."
Factfile:
Full name: Blaine Hansen Scully
Born: February 28, 1988, Sacramento, USA
Major teams: Leicester Tigers, United States of America
Position: Full back, Wing
Height 6ft 4in
Weight 205lb
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