Why are many African-Americans in Mississippi concentrated in the northwestern area?

T.E.D.'s answer (+1) is complete with regard to the human history of the Americas, but the rich soil in the "cotton belt" or "black belt" has a geological explanation as well. The article "How presidential elections are impacted by a 100 million year old coastline" by Craig McClain, "Executive Director of the Lousiana University Marine Consortium", describes recent research determining that this soil was the result of an ancient coastline where the skeletons of marine plankton accumulated.

The resulting chalk formations along the line of the old coastline underlie the well-drained, fertile soil. As elsewhere observed, this land was more recently preferred for plantations, and the descendants of some of the slaves that worked on them still live nearby.

You are describing part of what is referred to as the Black Belt in the American South. It can be seen here in a map of majority-black US counties.

Your suspicions for the reason for this are quite correct. This corresponds to the interior of the old cotton belt. This area was historically the richest river-fed southern soil, with the largest plantations, using the largest percentage of enslaved African-Americans. You'll note that for the north/south-oriented western portion, you can practically make out the outline of the Mississippi river.

After full-blown slavery was outlawed, this area largely set up a sharecropping system under the ownership of the same plantation masters that was as close to the old slavery system as could be legally countenanced. The workers were technically free, but living under a kind of debt peonage system where they didn't own anything, worked for someone else for a percentage, and would have trouble putting away enough money to afford to go seek their fortune elsewhere. Millions did anyway, of course, but the majority stayed.

The politics of this area and the states it passes through have been largely divided on racial lines since the Civil War ended. It would probably be most accurate to say that, perhaps due to the potential political power of that many black voters, there's generally been a dominant White Supremacy party wing in every state that has a large Black Belt component since the Civil War ended^*. Until the 1950's and '60's most of the black citizens in this area were not allowed to vote. The effort to realize their voting rights in the 1950's and '60's effectively destroyed the New Deal Coalition which the White Supremacists were part of.

This faction eventually landed in the Republican party in the new sixth party system, with the newly-enfranchised black voters almost unanimously going over to the other party, for what should be obvious reasons. This is how we got to today's party affiliation demographics in states like Mississippi, where about three quarters of whites identify or lean Republican, while blacks identify or lean around 6 in 7 Democratic.

I shouldn't leave off without mentioning the tremendous cultural impact this region has had on the greater US, and the world. Jazz, Blues, and Soul were invented here. Indirectly, Rock, Hip Hop, and Gospel trace their cultural lineage here as well. This means it is the cradle of most popular music (at least in English-speaking areas) worldwide. The African-American Vernacular English dialect originated here (which is why it is somewhat similar to Southern American English), before the post-Reconstruction diaspora spread it to large cities all over the US. This is the dialect of English used predominantly in all those forms of music, as well as the one seen in other US popular culture exports on TV and movies whenever a character is meant to sound like they hail from a tough (iow: poor) urban environment.

^{* - Obviously this gets controversial as we get into modern politics, and I'm really sorry for that. I'm not here to hurt feelings, but modern voting patterns were part of the question. Those evolved slowly from historical voting patterns, and historically southern political White Supremacy is an incontrovertible fact, literally encoded in law.}