Blair Underwood is scrubbing in on Fox’s “ Doc .” The hit drama, which just landed a Season 3 renewal, is celebrating that news by adding Underwood to the cast as a series regular. “Doc” viewers will first see Underwood as a guest star in the Season 2 “Doc” finale, which airs April 14. That will segue into a full-time presence as Dr. Ben Grant the following season. Here’s the character description of Underwood’s Dr. Grant, who appears to be on a collision course with lead character Dr. Amy