Fort Sumter was covered by a separate cession of land to the United States by the state of South Carolina, in a resolution passed by the South Carolina legislature in December 1836. The legislation did not specify that the Fort was to be used to protect the state. Fort Sumter belonged to the Unites States.
The Union blockades of Southern ports did not begin until *after* the battle of Fort Sumter. The Battle was in April, and the Union Blockade Strategy Board wasn’t even formed until June. The first shots of the Civil War were by the South—not the North.
The federal government could not and did not own state land.
If you underpin an argument of culpability on “who fired the first shot”, then that responsibility lies with the Union Army that fired on Floridians ninety days before Ft. Sumter.
The blockades at Pensacola and Charleston predated 4:15 a.m. April 12, 1861.