Whenever blacks become a majority there.
Actually, this makes sense. Blacks will likely become the majority in Mississippi in the 2040s, maybe even sooner than that. It'll happen sooner than most think, I think.
After eating my humble pie, I have to say: excuse me?
The proportion of Black people in Mississippi is currently growing at a rate of about 1 percentage point every decade. They are currently 38% of the population. What in the hell do you think will happen to make Mississippi a Black majority state within 20-something years?
They don't need to become a majority of the state to flip it though. If turnout equaled population proportion, with whites being 88% Republican (~2012 exit poll levels), and others at 3% of the vote, that would be 46.5% Democratic. Whites dropping to 85% Republican would be 48.34% Democratic and whites voting 82% Republican would be 50.11% Democratic. A coalition of composed of 98% of Blacks, 75% of Hispanics/others and 18% of whites would flip the state with a 59% White, 38% Black and 3% other electorate.