Where are you going to find the navy to blockade those ports? The Navy is mostly based in California and east coast blue states. It would fall mostly into the hands of the secessionists. Mexico and Canada would sympathize with the secessionists so blocking off the land border would be problematic. It is rather the inland core of red states who likely would become isolated and blockaded.
If those states were to go thru with seceding the Military I don’t think would go along...
Get the ships out of port and use them.
Not all of them would be out to sea, call the ones on deployment back to blockade the ports....
If you just blockaded LA and SFO that alone would cripple them..
Besides what countries would still want to trade with these 3 states if Trump or any other President told them it would be considered an act of war if they helped the 3 states....
My point is I don’t think it would be hard to cut those 3 states off from just about everything....