I've been thinking about this one all day,
Here's my take on it...
Robert Smalls was the pilot for the USN Ironclad USS Keokuk during the First Battle of Charleston Harbor. Now in OTL his piloting saved the Keokuk from sinking due to incessant Confederate Artillery Fire, only to have the vessel sink the following day.
Now let's have the battle go a bit differently, the commander of the Keokuk Alexander Rhind is killed by Confederate Artillery Fire along with a few other officers leaving Smalls in command of the vessel. In addition, several other Union vessels take significant damage causing them to retreat. During said retreat, the Keokuk runs aground on Morris Island and is stranded. The Confederate Naval Commander seeing his opportunity deploys his 2 ironclad rams in an attempt to press his advantage and perhaps weaken the blockade around Charleston.
Seeing the stranded Keokuk, the Confederate commander hopes to sieze the ship and salvage or capture her. With the Union Navy in dissarray, Du Pont orders the Keokuk scuttled. However Smalls ignores this order and instead directs the Keokuk's artillery fire against the Confederate Ironclads. This inspires a Union counter-attack and results in the reduction of Fort Wagner and the sinking of both Confederate rams. Furthermore thanks to Small's effort the Keokuk is saved and able to fight another day.
For his actions Smalls is awarded the Congressional Medal of Honor, becoming the first African American to do so. Though not able to translate this into becoming a captain, Smalls is able to utilize public opinion / his actions to gain entrance into the US Naval Academy. Yet he continues to serve as the Keokuk's helmsman for the duration of the Civil War.
Graduating after the American Civil War, Smalls serves in the US Navy with surprising competence, defying the racist claims of his detractors. Nevertheless promotion is hard to come by for Smalls who is consistently rejected in favor of other officers. However his example moves many more blacks to try and become officers especially in the navy.
After this it gets hard, perhaps an Anglo-American war in the 1890's leads to Smalls becoming a captain and then an admiral?