I don't know that it can be overstated. I was looking at newspaper reports, most of which pointed out that the untrained black troops fought as well as the veteran white regiment that was posted with them. Not only did it make national headlines, it made international headlines!
It meant that the fall of Vicksburg would yield access to new recruiting grounds with 200,000 - 300,000 men of military age, all of whom would "fight like tigers" as some reporters said.
Charles Dana thought it was remarkable enough to message the president directly.