In the darkest continuation of Marvel Comics' original Civil War storyline, Captain America's black shield still revealed his true loyalties. During the overarching universal reboot event that was 2015's Secret Wars, the conflict that ripped the superhero community in two over government registration was taken even further where America itself was split in two. That being said, Civil War: Warzones! saw a greater (and more tragic) resolution for Iron Man and as well as Captain America considering the state of the nation he had sworn to protect.
In Secret Wars, the Marvel Universe became the patchwork of mismatched regions known as Battleworld. One of them was the Warzone, a version of America where Civil War never ended, continuing for years after the battle in the Negative Zone which caused an explosion killing 15 million people with the region being split down the middle. While Tony Stark claimed the east and renamed it The Iron, Captain America's people took the west which became known as The Blue. The Iron is a place of law and order where superhumans were trained and monitored by SHIELD agents, while The Blue was founded as a territory of total freedom so long as its residents helped rather than harmed each other (lest they face the wrath of the Punishers). After years of war, the truth finally comes out that the conflict was exacerbated in secret by the Skrulls who intervened any time Stark and Rogers attempted to make peace.
Culminating in a final battle between the Iron and Blue in Civil War: Warzones! #5 from Charles Soule and Leinil Francis Yu, Steve Rogers led his forces wearing blue armor devoid of the classic white and red stripes. Likewise, his shield was painted black to represent the idea that there was no longer a united country for him to represent. However, the black paint gradually scratched off as the battle went on, revealing the red and white stripes underneath, proving that deep down Rogers desired to see his nation made whole again.
Following an epic fight with Stark, the truth about the Skrulls playing both sides was revealed, a revelation that saw Iron Man and Captain America working together to end the war and reunite the country (giving their lives in the process). All in all, the visual representation of Rogers' black shield is a powerful visual symbol, serving as a great metaphor for Steve's loyalties. While he initially appears to have chosen one side and looks ready to destroy the other, the Sentinel of Liberty proves that he's still for all of America and its people as a whole the second the chips were down.
When you scratch the surface, Captain America's loyalties are always a constant. It's why he ultimately surrendered in the original Civil War storyline. For all his pride and stubbornness, he knows when to back down if his fight isn't in the interest of the American people and its core values. While he was initially fighting for the rights of his fellow superheroes, he eventually saw that open war and continuing the status quo wasn't in the interest of the nation and what it ultimately needed. On a grander scale, the same is true here in the Warzone with Captain America's black shield being the perfect symbol for the epic Marvel event.