African-American % by state, by decade

[True Federalist (진정한 연방 주의자)]

So, you guys have heard of the Great Migration, right? It's an important thing. Especially critical to keep in mind when thinking about Reconstruction.  Also, slavery was banned from the Great Lakes states way back in the Articles of Confederation days. So, that's why outside of the big Industrial cities, there are still no black people in the farm country up that way.

There's also the fact that before the Civil War, quite a few of the Old Northwest states had laws that barred free blacks from settling there as well.

[True Federalist (진정한 연방 주의자)]

1850:

Kind of weird that Washington is over 10% black here, drops massively to less than 1% in next census.

Not really.  One part of the compromise of 1850 was the banning of the slave trade in the District.  So in the 1850 census, it's quite likely a number of the slaves showing up on the census rolls would have transients who were there on census day simply because there were in the process of being sold.  Incidentally, the prospect of the banning of the slave trade was a major impetus behind the retrocession of Alexandria to Virgina in the 1840s, as Alexandria also had a thriving slave market.  That retrocession probably explains the drop for DC from 30% in 1840 to only 10% in 1850.