They use this blue garbage bag for cheering up their team.
Japanese supporters were very disappointed after being pipped at the post, but even after the loss, they started cleaning up the place with blue garbage bags in their hands.
In fact, it is a traditional event which has been done for 16 years since Japan’s first participation to the (soccer) World Cup game.
We have lost at the first league in the World Cup in France in 1998, but in every single match, Japanese supporters cleaned all the areas including where opponents were sitting.
— World Cup Problems (@WorIdCupProbs) 2014, 6月 15
Many medias in the every countries are praising Japanese supporters.