When Rockstar, David Bowie, died two years ago at the age of 69, he left behind a parting gift. He’d been secretly working on a new album, knowing it would be his last. “Blackstar” is Bowie’s message from the great beyond. The singer’s death, and his last work, so moved a Boston based musician that he composed a symphonic homage to him. The group called, “The Ambient Orchestra,” will perform it Friday Night at the Moss Arts Center in Blacksburg.