The north’s blockade of the southern sea ports was challenged in an early battle at sea between which two iron-clad vessels? civil war

The battle you are referring to is actually called the Battle of the Ironclads. In this battle, it was the Merrimac (from the Confederacy) versus the USS Monitor (from the Union). This was the first battle between iron-fortified vessels in the Civil War. This battle ended up in a draw but represented a new form of fighting that both the Union and Confederacy had to be ready for.

C.The Monitor and the Merrimack