Final answer:
Blacks crossing over to Union lines after victories in the South is true due to their critical role in the Union's success, especially highlighted after the victory at Vicksburg.
Explanation:
True, after Union victories in the South, blacks would cross over to the Union lines. This occurred following important moments like the victory at Vicksburg, where President Lincoln recognized the significance of black soldiers in achieving Union victory.
Learn more about Union victories and black soldiers during the Civil War here:
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