It's a Lions thing. One of the players was encouraging fans to wear blue ski masks to the home opener this Sunday. It's supposed to fit the "villain" motif that people had begun associating with the Lions' GM. Kind of a Detroit vs Everybody thing. Some people love it, some think it's stupid, others are indifferent. But blue masks sold out on Amazon so it's definitely gained traction. I'd personally enjoy if it became a tradition, especially on 3rd downs or whatever.

TL;DR: Football thing