Here's why:
* The show's costume designers often create unique pieces. Many of the characters' clothing on Blue Bloods is specially made to reflect their personalities and roles.
* Selleck is known for his particular tastes. He's a man who appreciates quality and often prefers custom-made items, especially when it comes to hats.
* The hat is a signature part of his character. The fedora is an iconic part of Frank Reagan's look and is a key visual element that contributes to his authority and personality.
While the exact brand might remain a mystery, it's safe to assume that Selleck's fedora in Blue Bloods is not an off-the-rack hat.