Final answer:
Black people were allowed to join the Union Army after the Emancipation Proclamation on January 1, 1863, with nearly 200,000 black men serving by the end of the Civil War. The Militia Act of 1862 was the first official authorization for African Americans in federal military service, leading to the establishment of the Bureau of Colored Troops.
Step-by-step explanation:
Black people were officially allowed to join the Union Army after President Abraham Lincoln issued the Emancipation Proclamation on January 1, 1863. This followed the earlier Second Confiscation and Militia Act of July 17, 1862, which was the first official authorization for employing African Americans in federal military service. African Americans, both free and formerly enslaved, made significant contributions to the Union's war effort, and by the end of the war, nearly 200,000 black men had served in all-black Union Army regiments.
Lincoln's initial hesitation to employ African American troops was due to concerns about the loyalty of slaveholding border states and the willingness of white volunteers to fight alongside black soldiers. However, as the war progressed, the actions of African Americans and their crucial role in the war effort compelled Lincoln to recognize the necessity of their inclusion in the military and, eventually, to consider policies towards emancipation.
The Bureau of Colored Troops was established with General Orders No. 143, which allowed for the recruitment and training of African American soldiers. It's noteworthy that no blacks were officially accepted into Confederate military service until the final months of the war, and even then, the recruitment was minimal and without significant impact on the Confederate forces.