Rosenborg's superior fitness began to tell as the half progressed and Roar Strand blazed wide before Blayney brilliantly tipped Henriksen's shot round the post for a corner.
However, Andy Graham missed a glorious chance to extend Newry's advantage on 62 after finding himself with only Blayney to beat but the Linfield keeper raced off his line to block.
Garrett should have killed off Ballymena on 69 minutes after a poor Blayney clearance but the keeper blocked the striker's pointblank effort and Gary Haveron stopped the follow-up effort.