The only problem is that midwest farming, not to mention northern midwest, is quite different from the south. Especially given the cold, if the government does not offer massive assistance to the resettled ex-slaves, they are not going to prosper and may suffer severe attrition. As the poster before me mentioned, the German and Scandinavian settlers in the area had enough money and experience farming in similar climates with similar crops to afford to make a go of farming in the northern midwest. So the question is if the US government wants to make the investment to settle ex-slaves, or just let the south deal with it.
As it is, they could just let the northern European immigrants have it for free and develop it at their own cost. If the abolitionist organizations want to shoulder the cost of settling and equipping ex-slave sharecroppers, the west would be warmer and probably a bit more like the south as far as farming goes. Though, we should note, the southwest is going to be half in the USA, as the Confederacy did not take this area or settle it by the time the war happened. In fact, place like Colorado and Utah were firmly in the north during the civil war.