Value of the double-blank domino?
I am not sure what you mean by a standard connect game (it is also odd that you tagged it with trick-taking-game. There are variations of dominos that are trick taking, but you don't make connections in those games). Nearly all variations of Dominos have you count the number of pips on dominos left in a losing player's hand. Since the double blank has zero pips, it is worth zero points.

Draw Dominos


  Scoring - When a hand ends, the player with the lightest hand (i.e. the fewest number of dots on their dominoes) wins the number of sum total of points in all of his opponents hands (minus the points in his own hand, if any).