Early on the Union blockade was essentially a paper blockade, and therefore of dubious international legality. There were laws regarding blockades, but those laws were fuzzy when it came to civil wars & a nation blockading their own ports. The Union would have been in violation of international law if the Confederacy were a recognized country, which would have given Britain and France a pretext to end the blockade by force. Early in the war, almost any European sea power (Britain especially) could have broken the Union Navy in a matter of hours.
The Confederate government imposed a cotton boycott to trigger economic turmoil in Britain especially (Britain was always their primary diplomatic target), which they felt would create enough political pressure in Parliament to spur recognition and a complete lifting of the blockade by the Royal Navy.
The economic problems in Britain were very real (triggering waves of emigration & from Lancashire & other towns that were cotton dependent) but the British largely worked around them. They retooled their factories to work with cotton from Egypt, India, and the East Indies. Workers found other jobs. Government relief. A lot of people suffered though.
The Confederates seriously underestimated the degree to which Britain and British politicians loathed slavery. Slavery stacked the deck against them diplomatically from the beginning. There were other geopolitical reasons (like Britain not wanting to risk Canada by sparking a conflict with the Union) but the British basically chose to suck it up and endure the cotton famine & economic depression because they wanted slavery gone, and America was one of the last major holdouts at that point.
I've generally had a pretty cynical view towards British abolitionism, which is that the elites only disliked slavery to the extent that they found capitalistic imperialism (i.e. systems that controlled the lower classes through persistent indebtedness and restrictions on ownership of real estate) to be more profitable and politically stable than the systems constructed on the institution of slavery.
And, coincidentally, that their empire, whose 19th century wealth was bootstrapped on the profits of slave-trading and the manufacture of raw materials extracted via slave labor, was uniquely-well-positioned to come out on top in a world order deprived of slave labor.
In other words, they opposed slavery on the basis of limiting economic volatility, and maintaining their geopolitical status, more so than any moral basis.
I'd love to read more sources that balance out my cynicism. :)
> I've generally had a pretty cynical view towards British abolitionism
I am not an expert on abolitionist movements or their political effects, but there was a strong moral outrage to slavery in the 19th century and it developed very rapidly, starting in Britain. I recommend reading about the British public's reaction to the publication of Uncle Tom's Cabin.
> their empire, whose 19th century wealth was bootstrapped on the profits of of slave-trading and the manufacture of raw materials extracted via slave labor, was uniquely-well-positioned to come out on top in a world order deprived of slave labor
You're not wrong, and you can toss colonialism in there, too. The Japanese were made the same argument in the early 20th century as they were trying to expand and acquire colonies and kept getting their knuckles whacked by the Europeans, who were (mostly) trying to pump the brakes on colonialism by that point.