A Spool of Blue Thread question

The first year Denny doesn't go with them to their vacation house, they hear a French Horn being played. That was the instrument that Denny played, so they thought he might have been near. Red is asking if he paid the person who played that instrument during the trip.

Yes he was alluding to the fact that Denny use to play the French Horn.

The blue thread I think was to mentioned near end of book where Denny had to sew something together

The blue thread used to repair the blue shirt was the same spool of blue thread used to make the clothing Abby and Red wore on their wedding day. Red wanted to wear the same shirt to Abby's funeral, so it had to be repaired. I see the blue thread as a connector of their love throughout their lives.

I think he realized that his son was manipulative, and it suddenly dawned on him that the French horn player who parked outside their beach house (the year that Denny wasn't there) might have been a set up - paid by Denny.

I am confused. What was the significance of the blue thread?