In looking at the the census figures for 1790, what is interesting is that the overall populations of the "free states" (that term really doesn't apply in 1790 - but by this I mean the states that we think of as being the free states) and the slave states is almost identical. The free states had a population of 1,968,154 and the slave states had a population of 1,961,172. It would never be this close again; however, it wasn't until the 1830s with the increase in immigration, that the free states really started to pull away in terms of population. Also, the percentage of the population that was slave in 1790 was 17.8%. Of course the percentage of slaves in the slave states was probably close to 35% at that time.
What is also interesting is that both New York and New Jersey's populations in 1790 were 6%+ slave. That figure just blew me away. I had no idea that so many slaves existed at that time in these "free" states. Of course over the next 20 years, slavery had pretty much been abolished in the free states. But, if slavery had been more prevalent in these free states in 1790, say something like 15% of the populations of NY, NJ, and Penn were enslaved, we could have seen a much larger free black population in the North during the first half of the 19th century - provided that these free states still emancipated their slaves.
What if the free black population of the North in 1860 was 10 - 15% of the overall population? How would this demographic changed history?