The economic numbers demonstrate otherwise. The Union DESPERATELY needed to stop that loss of trade and the arisal of competing industry.
Northern financial positions were under a massive threat from Southern Independence, and they simply could not allow unimpeded trade to develop with Europe. They also could not allow for the capitalization of competing Southern industries that would develop from the increased economic benefits of free trade with Europe.
Northern industries would have been severely attenuated and/or possibly destroyed. The balance of economic power would have shifted to the South, and the Empire State simply could not allow such a thing to happen.
This is why the first thing the Union did was to throw up a blockade. That trade had to be stopped at all costs. If trade had been allowed to be established, and if the Europeans had become accustomed to receiving the economic benefits of free trade with the South, the Europeans would have used their military might to insure that it continued unimpeded.
This was a war about money, despite all the subterfuge intended to divert attention towards everything except the money involved.
That would be comical if it weren't so preposterous.