In Japan, some amateur historians speculate that, since the 'Kokushi-koku' was known as another name of Japan in Korea and China at that time, the Baekje royal family members were sent to Wakoku (Japan) and stayed there for a long time (assuming that the hostages that appeared in the Nihonshoki [Chronicles of Japan] included these Baekje royal family members), and as these royal family members brought back a Japanese habit of painting one's teeth black, Baekje people called them the Kokushi (literally, "black-teeth") family, which gradually became a widely-used name in the country.
また、日本では、一在野の史家の論ではあるが、「黒歯国」は当時の朝鮮や中国などでは日本の異称として知られていたことから、倭国（日本）に派遣されて、長期滞在（日本書紀などに見られる百済の王族などの人質などを想定）し、それらの王族が帰国後、倭国の文化であった歯を黒く染める風習（お歯黒）を百済に持ち込んだために、百済の人々から「黒歯」氏とよばれ、それが定着したとする論もある。