I think it was the use of force that was monetized. Corporations required willing workers, but willing workers enabled higher productivity. It did take a while for people to realize that.
I wonder how black owned corporations were able get capital before the war to buy slaves (to set them free), but corporations were unable to borrow money after the war?
Part of it may be that with the government borrowing money to finance the war, interest rates were higher.
Some part of it may have been KKK activity, that threatened anyone who did business with freedmen.
My understanding is that most of the "black-owned slaves" were family members bought by free blacks who were not then allowed to free them because of restrictive state laws on emancipation.
I'm really unclear how "corporations" are supposed to fit into this picture. My understanding is that in the first half of the 19th century corporations were few and far between. With no general enabling laws, setting one up usually required a special act of the state legislature, which in southern states doesn't seem very likely for blacks wanting to free slaves. Most business was carried on by sole propietorships or partenerships, not corporations.
The whole issue of whether slavery made sense economically is usually misstated.
Regardless of what some have stated, slavery was wildly profitable for planters. In a good year they made enormous amounts of money. (In bad years, they blamed their troubles on northerners.)
The costs of slavery, and they were enormous, were born by the society as a whole. In essence, slavery was a way of privatizing the profits and socializing the costs of the institution.