The Bible verse containing the phrase "blood toucheth blood" is Hosea 4:2.
In the final scene where Jimmy Perez (Douglas Hanshall) says to Duncan Hunter (Mark Bonnar) "This could be the beginning of a beautiful friendship". It is an obvious nod to the final scene in "Casablanca" between Humphrey Bogart and Claude Rains. Which also take place in an airport.
Mark Bonnar (Duncan Hunter) & Leanne Best (Annabel) also worked together on Undercover (2016) as John Halliday & Abigail Strickland respectively.
Julie Graham (Rhona Kelly) & Anne Kidd (Cora McLean) also worked together on Retribution (2016) as Moira Douglas & Meredith respectively.