And why would America start a war with the CSA when they it's allied to France and the UK? That's suicide. And if America was bone headed enough to start a great power war the peace would have been far more onerous than OTL because they would have an eye on the horizon for America joining up with a foreign power and be looking to knock America down a peg.
I'm going to defend Blaine on that one. To get British intervention on behalf of the Confederates, you have to have:
1) the British going to war with their primary source of food imports in order to support an expansionist slave state. (An eventuality that would seem extremely improbable to any objective observer); and
2) the Confederates being willing to voluntarily give up slavery just so they can acquire Sonora and Chihuahua. (An eventuality that would seem outright impossible to any objective observer.)
Both of the pre-conditions necessary for British intervention are so incredibly unlikely, that I really can't blame Blaine for not considering British intervention to be a realistic possibility.
I don't think the bread issue would be as dire as it sounds. Western Canada was opening up, Russia had exports, the CSA will have some, and Britain was a very wealthy nation.
Western Canada would be in the middle of a war with the U.S. which might limit grain production somewhat. There are serious geopolitical problems with the British going begging to the Russians for a million tons of grain. (Are the British really going to be willing to give Constantinople and Persia to the Russians just so the Confederates can acquire Sonora and Chihuahua?) And the Confederate states were never large grain producers and will also be in the middle of a war which is going to further reduce their grain crop.
Anyway, I think you are underestimating just how much grain the British were purchasing from the U.S. in the 1880s.
Per the Institute of Economics at the University of Copenhagen, 62.8% of the British population was dependent on foreign wheat imports in the 1880s.
http://www.economics.ku.dk/research/publications/wp/2004/0428.pdf pg. 8
And more to the point, the UK's biggest source for wheat in the early 1880s was by far the United States. Per Michael Atkins, The International Grain Trade, Second Edition, pg. 18, for the period of time from 1878-1882, the UK's annual average of wheat imports from the Untied States was 1,753,000 tons. During that same period the UK's annual average of wheat imports from Russia, Germany, Canada, India, Australia, and Argentina
combined was 1,026,000 tons.
https://books.google.com/books?id=4...VSwCm8Q6AEILDAA#v=onepage&q=1,753,000&f=false
So if 62.8% of the British population is dependent on foreign wheat imports in the 1880s, and the UK is getting more than half its wheat imports from the United States, then 1/3 of the British population is dependent on US grain exports.
That's a staggering amount of grain which is going to be very difficult for the British to replace. Best case scenario still sees the price of grain in Britain skyrocket which makes the existing government very unpopular. Worst case scenario sees widespread starvation in Britain which will inevitably lead to revolution at home and abroad and probably ends with Queen Victoria being guillotined by some London Soviet.