1810:
Not sure what the huge drop in Alabama is all about.
That is for the portion of Mississippi Territory that is now in Alabama. In 1800 there were around 1000 persons, and in 1810 about 9000. The southern part of the state (West Florida) was not in the USA at the time. In 1810, Madison County (one of three counties) had half the population, which would have been settlers pushing down from the mountains of eastern Tennessee. For a slave economy, you need a way to transport whatever is produced, whether it is cotton, tobacco, sugar, or indigo. For subsistence farming, you only need a way to get there, and perhaps enough population to provide security from the people you are taking the land from.
1850:
Kind of weird that Washington is over 10% black here, drops massively to less than 1% in next census.
Washington portion of Oregon territory. 152 of 1200 persons, and the census did not distinguish non-black non-whites until the 1860 Census. This might have included some Indians, black fur traders (such as Jim Beckwourth), and perhaps some Chinese. In 1860, non-black non-whites only constituted 0.25% of the USA population.
Incidentally, Columbia County, New York may have had its maximum black population in 1800. In later censuses, the outlying towns were mostly slave, while in Hudson they were mostly free colored (a not atypical pattern that you will also see in places like Baltimore, Charleston, Richmond, New Orleans, and other cities among areas that had slavery.
