According to the RIAJ, or Recording Industry Association of Japan, an album needs to sell over 250,000 copies to be certified "Platinum" and below that is the "Gold" certification, meaning that an album needs to sell over 100,000 copies. The RIAJ website was recently updated showing that "Blood, Sweat, and Tears" was certified Platinum!
It's rare for a Kpop artist to achieve Platinum in Japan and when they do, it can take many months to achieve it. However, for BTS, they achieved it in just a few weeks! It appears their popularity is rapidly rising in Japan as well.