What would happen if Mississippi became 50% black?

[I spent the winter writing songs about getting better]

He probably meant counties.

[I spent the winter writing songs about getting better]

It's worth noting that Mississippi is about 4% neither white nor black, so in a 50% black, 46% white and 4% other demographic, the Democrats definitely have the advantage even if the other vote is about 50/50, which it probably isn't. Also let's compare two counties:

Attala County
Romney 5,126   56.38%
Obama  3,927   43.19%

Yazoo County
Obama 6,603   56.82%
Romney 4,941   42.52%

Alright, so pretty close to mirror images in voting percentages. Now demographics:

Attala County
56.2% White
42.0% Black

Yazoo County
44.74% White
53.96% Black

And finally population:

Attala County 19,564 (~46% turnout)
Yazoo County 28,065 (~41% turnout)

So based on that I'm sure someone could calculate turnout rates for both whites and blacks, but I'm too tired to do it now. However it does appear that the 4% other would prevent it from being a true pure swing state.