Final answer:
The Confederate recruitment of black soldiers exposed a fundamental contradiction in proslavery ideology, as it demonstrated a tacit admission that African Americans could serve capably in roles previously deemed unsuitable for them, thus challenging the slaveholding society's underlying racial justifications.
Step-by-step explanation:
The planned recruitment of black soldiers for the Confederacy undermined proslavery ideology by contradicting the foundational principles of the Confederacy that African Americans were unfit for duties that required loyalty and intellect, such as serving in the military.
This act of desperation indicated that the Confederacy was willing to compromise its core beliefs for additional manpower, thereby acknowledging that if black individuals could be good soldiers, then the entire justification for slavery was flawed. Such an action weakened the Confederate cause by highlighting internal contradictions and underlined the strategic and moral importance of black soldiers in the Civil War.