Final answer:
The captured Union ironclad ship that the Confederates hoped to use to break the Union blockade was the USS Merrimack, rebuilt and renamed as the Virginia.
Step-by-step explanation:
The captured Union ironclad ship that the Confederates hoped to destroy the Union blockade with was the USS Merrimack, which they rebuilt and renamed the Virginia. The Virginia was one of the most famous ironclad ships of the American Civil War, and it played a significant role in the Battle of Hampton Roads in 1862. The Confederacy saw the Virginia as a means to break through the Union naval blockade and open up Southern ports for trade and supplies.