Blair, whose uncle, Terry, was convicted in 2008 of murdering six women, is already serving a 23-year sentence in prison for other convictions.
Blair's mother was also convicted of murdering her boyfriend in 1989.
Even before Terry Blair was convicted, a lawyer representing Diamond Blair's brother in a murder case said his client was "born to a family of murderers."
Friday, the jury found Blair guilty of murder, robbery and armed criminal action in the death of Montague Kevin Ashline, 22, in Kansas City in June of 2009, The Kansas City Start said.
Court records saw that Blair shot and killed Ashline during a robbery attempt.
Before the incident, Blair appeared in juvenile court for assault between the ages of 12 and 16 for car stealing, other robberies and kidnapping and weapons violations.
At the age of 16, a judge said he was fit to stand trial as an adult and sent him to jail, where Blair and two others sodomized a 15-year-old boy.