I think many of the men were from the slave state of Missouri. Don't forget that some men- maybe quite a few, or quite a lot- and that both Black and White men- served in regiments they didn't live in.
How did Iowa raise a black regiment???
There have been several answers to this and the original post and I think they're all good, but I think one more comment can be added that's relevant, not just to this question, but to our general understanding of the USCT and emancipation.
Let's start with the basics. Iowa in the 1860 Census is reported as having a Black population of 1,069 but the BCT credited it with recruiting 440 USCT. But, also in the Census, men of military age were assumed to constitute 20% of a state's population, while overall the average of the population for all northern states serving in the military was closer to 10%. So likely only around 100 of the 440 credited to Iowa were actually living there before the war started.
The balance of the 440 credited to Iowa would have come from Black men who made their way to the recruiting rendezvous at Keokuk from other states, along with those recruited by authorized state agents representing Iowa. And this may have represented all of the six companies raised in Iowa. Their subsequent service took them to Helena Arkansas and other hell holes in the lower Mississippi which would not only explain their disease rate, but also that most of the additional 700 men came from Arkansas, Louisiana, etc.
Two conclusions I draw from this are, first, it reinforces our understanding that most enslaved people were not passively waiting to be emancipated -- most were *self-emancipated and traveled some distance for the freedom offered by the United States flag.
Second, a large portion of USCT regiments appear to have been more cosmopolitan than white volunteer regiments, being a combination of those free or enslaved before the war and representing a variety of communities, urban and rural, plantation and small farms, covering broader regions, all under an overlay of white officers also geographically and culturally diverse -- in other words, as much "United States" as "Colored."
Something to keep in mind when we see the letters "USCT" -- each one of those regiments has their own unique story, complex and diverse.