It was blackout day for the game.
There is a legit reason when it’s for a sporting event. Totally common at a lot of games. Fans are encouraged to wear the team color to show support and many kids (and adult fans) will also use paint to completely cover themselves. My hometown college team’s colors are red and black. Sometimes they do red, sometimes they do black. Nothing racist about it, it’s all about boosting team spirit.
Many stationary pads have lines like that.