I expect textbooks just need to save space but the blockade was definitely significant to the war. Supply problems plagued the Confederacy throughout the war, due in no small way to their inability to trade and supply themselves effectively through international trade. Of course, it took some time to procure the ships to actually blockade the South, so it wasn't fully effective from day one.

The blockade was also significant because of its effect on Union/Confederate international relations and the possibility of other countries like Britain and France recognizing the nationhood of the CSA (which never formally happened in the end). Lincoln refused to acknowledged that the Confederacy was no longer part of the United States, yet the act of a blockade was reserved for enemy nations under international law at the time. For the first couple years of the war (when recognition was most likely), the US was put in a peculiar situation diplomatically by both refusing to recognize the CSA as a foreign nation but also refusing to give up the blockade.

Finally, I think people too often mistake the blockade as something ineffective or foolish, pointing to successful blockade runners as evidence that Confederate trade still took place. What they tend to ignore is that blockade runners are small, fast ships not built for large amounts of cargo, were periodically caught, and that blockade runners could never hope to feed and supple an entire nation like the CSA. The blockade stymied Confederate trade on a massive scale. Instead of thinking solely about blockade runners, think instead about how many trade voyages never took place due to the blockade, despite the Confederacy badly needing supplies (the voluntary cotton embargo by Confederates also contributed of course).