When in doubt, start with a TV icon. That’s what the producers of “Blue Bloods,” debuting at 10 Friday on CBS, did. They choose Tom Selleck as the police commissioner who heads up a family with three generations of “New York’s Finest.” They then surrounded Magnum with such familiar TV faces as Donnie Wahlberg, Bridget Moynihan and Will Estes playing his children (the men are cops, Moynihan is a district attorney). Then they took an interesting family dynamic and wove in a mystery about how the third brother, also a cop, died. “Blue Bloods” is getting every chance to succeed. It skews old, making it a perfect fit for a Friday night, and it’s up against another rookie, “Outlaw,” the Jimmy Smits starrer, which isn’t nearly as well developed.