The America you theorized would probably have been a reason for World War 1 happening a bit sooner.
What people don't realize is prior to WW1, Europe was in a long period of sustained peace and prosperity since the end of the Napoleonic Wars. Although there were battles and tension, shyt was relatively peaceful. (Interesting enough, I know many people talk about how greedy europeans are but, in actuality, most of the populations didn't understand enough to care about the colonies that were being established abroad. The greed was really all the aristocracy.)
What also was happening was European leaders were literally playing monopoly with the continent of Africa and they were licking their chops at jumping on the territories of the Ottoman Empire because of its impending collapse.
Now, if North America was a sovereign territory of mainly blacks, that could have looked very different depending on the trajectory of established society.
- Would the Black population have just become tribal much like our ancestors and the native Americans to the West? If so, that North American territory would be like free lunch for the numerous world powers ready to colonize and claim resources.
- Would the Black population have established its own unified government? If so, we'd have had bloody battles and wars with the Native Americans while trying to take over territory and resources in order to sustain a large society that can protect itself from foreign wolves.
- Would the Black population have established various nations across North America? If so, the coastal nations would be vulnerable to foreign influence and possible invasion by greedy world nations trying to land grab.
I believe those European nations would have probably engaged in War prior to 1914 as a result of many of the tensions + the added tension of "who's going to conquer the American territories first."
It sucks but considering Native Americans were too tribal and traditionalist to agree to join and assimilate into a Black American government, I believe we would have adopted a lot of the imperialistic characteristics that are attributed to Euros on the way to establishing a unified "Black United States."
It would have still been cool from my perspective of a Black man in America but we'd have joined that alternate timeline's category of "Imperialistic societies."