It's not hard to contrive a "what if" scenario, but it does take some construction. I don't know if something centered around the Cleburne proposal would work out as reasonable speculative history, but there are other things one could do.
Historians tell us that nothing is inevitable. People respond to events and other people, and the nature, scope, and timing of events make a difference. Historians use the term Contingency to conceptualize the idea that history unfolds "in response" to events.
{Contingency is key to historical thinking and helping students understand that while in hindsight, the past seems to unroll in logical storylines, this was not necessarily the case for those who lived through it. If suffragettes hadn't taken to the streets in the 1910s or focused on changing state laws, would they have won the vote in 1920? If the Treaty of Versailles contained different stipulations, would Germany have taken the path it did and would WWII have happened? Historical events are dependent (or contingent) on multiple causes that shape when, how, and why an event happened the way it did.}
Looking at the history: The Confederacy supported slave enlistment at the start if 1865, when it was clear the CSA was losing, and in fact, close to defeat. It seems that slave enlistment was
contingent on the CSA being close to defeat, and desperate enough to use slaves as soldiers.
But
what if desperate times occurred sooner than that? We know that in real life Confederate forces in Virginia enjoyed success against Union forces early in the war. Suppose that Confederates were suffering bad losses instead? Suppose those losses were so terrible that R E Lee ~ who championed black enlistment in 1865 ~ supported black enlistment in 1862? He might have been able to convince his home state of VA to create black militia units to give armed support to Confederates (and state officials did finally support using slaves as soldiers in early 1865).
Thusly, Lee might have started by using free blacks as soldiers - Virginia had 58,000 free blacks, the most of any Confederate state (44% of all blacks in the Confederacy resided in VA). Let's suppose that those black soldiers were decisive in a battle or 2 or 3 or more. That could then lead Lee to use even enslaved men as soldiers.
NC is right next door to VA, and it had the second most free blacks of any Confederate state. It could have followed the example of VA, and then used free blacks as soldiers, and then used enslaved people.
And following the precedent of those two states, maybe more states enlist slaves. (There were very few free blacks in the CSA outside of VA, NC, and LA.) Maybe.
That's just an example of how you could contrive a reasonable scenario for the employment of black soldiers.
- Alan