One, the soldiers in the photo are Americans. The cut of the uniforms (high collared-tunic with pointed breast pocket flaps) fits the American uniform in 1917-1918. The cartridge belts with ammunition pouches are also American, the Canadians carried their ammo in web pouches on their chests, as you can see here

Two, they're probably a rifle platoon (~60 men in the U.S. Army of WWI), since they have rifles and steel Brodie helmets (the famous broad-brimmed "tin hat"). Most black troops in WWI were used in labor units and many men in labor units wouldn't have been armed or given helmets.

Three, there were quite a few black units in the U.S. Army during the war.

At the start of the war, the 9th and 10th Cavalry Regiments and the 24th and 25th Infantry Regiments, the famous "Buffalo Soldiers," were all part of the Regular Army, although none of them saw combat. Instead, they did garrison duty along the Mexican border, in Hawaii, and in the Philippines.

The black soldiers who saw combat were volunteers, draftees, or National Guardsmen from northern states who'd been called up. Although many of these men ended up in support units, some did see combat. There were two large frontline formations that were mostly black: the 92nd Infantry Division and 93rd Infantry Division. The 92nd fought under U.S. Army control and had four infantry regiments, three field artillery regiments, a trench mortar battery, three machine gun battalions, a signal battalion, an engineer regiment, an engineer train, and various support units. Most of the junior officers were black, although senior officers were white. The 93rd was attached to the French, since some white American troops refused to command or work with the black troops. Also, the French were clamoring for manpower, so the black soldiers were a much-needed and welcome reinforcement. The 93rd had four infantry regiments, three of which were comprised of National Guard units from New York, Illinois, Ohio, Maryland, Connecticut, Massachusetts, the District of Columbia, and Tennessee.

You can read more about how the two divisions faired in this excellent article.

Is there any chance of getting a higher-resolution scan? The more I can see, the more I can tell you!