A series of laws and court cases helped disenfranchise blacks and allow for segregation.
For example, the Plessy vs Ferguson was Supreme Court case that allowed for segregated facilities in society. The Supreme Court judges ruled that as long as the facilities are “ separate but equal” then they are allowed.
Besides that, the southern state governments passed laws like literacy tests, poll taxes, and the grandfather clause were put in place in order to stop black citizens from voting.