At any hoop, when the opponent has a ball in scoring position while both of your balls are far away, your choice for your first ball depends on how easy a shot the opponent has to score, and whether it is an odd or even hoop. In general the worse the opponent's position the more attractive it is to try to block with your first ball. However this is dangerous at odd-numbered hoops, because if the opponent's second ball gets good position the first ball can now clear you the length of the lawn.
At any rate, if the opponent has 18" straight position with the first turn of the game, you don't have to think. Try to clear with red. If that fails, try to clear with yellow. No matter how unlikely you think you are to succeed at the clearing shot, you are deluding yourself if you think your chance of blocking blue is anywhere near as good as your chance of clearing it.
Any concerns about blue going in the jaws if both balls miss and are sitting on the west boundary?