pure speculation.
The numbers support it. Even if it was speculation, the Northern Newspapers of the time repeatedly voiced it, and likely a lot of Northern people believed it.
Certainly the Robber Baron types believed it, and this was sufficient to convince these people that a war to stop the South from trading directly with Europe was necessary to protect their financial interests.
Blockade was nearly the very first thing they did. Blockade runners could get weapons through, but slow moving cargo ships certainly couldn't get shipments of Cotton or Tobacco through it.
Not much of a benefit to fighting a war, but absolutely essential to prevent the South from establishing trade with Europe.