Japan has always taken in a very small number of "refugees" each year. The ones who get their hooks in and stay can then recommend more people to bring over. For example there are NGOs, often churches, targeting African immigrants that want to escape the Congo or wherever.

Black people specifically are given jobs as street hustlers at red light districts and Harajuku because they are aggressive and young girls think it makes the shops look cool.

Most of the sandniggers were brought in during the recent international campaign for all countries to take in "refugees" from 2012 to 2018. They joined existing expat communities and occasionally have these pissy little turf wars. Japanese people are annoyed by it but don't get outraged unless it starts inconveniencing them. Like in every other country on earth they work at convenience stores and sell kebabs.