Trying to read Storm of Steel and Junger says: "The shells with black smoke, which the old-timers called 'Americans' or 'coal boxes', ripped with incredible violence."
I searched Google and the only thing I discovered was that Americans called artillery shells "Jack Johnson" after the famous boxer. It would be pretty fascinating if he was so renown the Germans used the same term for the same reasons.
The name comes from the fact that so much of the Allied ammunition was manufactured in the United States, even before 1917. As Keegan writes in his The First World War, Britain was manufacturing 3,000 rounds of field-artillery ammunition per month when the war broke out in 1914. A single 18-pound gun could fire 10 rounds (or more) in a minute of bombardment. You can do the math and see the problem. By April 1915, the British had improved production to 225,000 rounds per month, but that still wasn't enough to meet demand. American purchases filled the gap for the British, French and Russians alike.
By 1913, the American economy was already the largest in the world, producing a full third of the world's manufactured goods. Howard Blum's Dark Invasion is a popular history of the sabotage and misinformation campaign the Germans operated in the United States to counteract that enormous productive capability. "We are at our wits' end to defend ourselves against American ammunition," Blum quotes the German Supreme Army Command in an early 1915 telegram to Berlin.