In the case of a tie vote on a bill, the vice president has the casting vote.
Explanation:
A casting vote is a vote exercise by someone, in this case, the vice president, to decide over a tie on a legislature body. The vice president only votes in that case, if not he/she only presides the sessions of the Senate or joint sessions of the Congress.
While the vice president’s role as president of the Senate is largely ceremonial, in cases of a tie vote (50-50) on a bill, the vice president can break the tie.
The answer that can be concluded from the Unions desire to blockade ports in the South were that foreign nations were willing to trade partners with the confederacy.