Cotton was the United States largest single export, at a time when agricultural exports accounted for 75-80% of all exports, so this is going to have a negative effect on the United States balance of payments and could result in a short-term economic crisis where hard currency (gold/silver) are hoarded. New York City is going to be negatively impacted as well, due to it being the hub of the cotton trade.
In 1833, the United States reduced tariffs with the Tariff Act, this made imported goods (primarily manufactured goods from Britain) cheaper. American industry was still relatively small, and tariffs were reduced until the Civil War, when agricultural exports jumped to 81% of all exports, due to the increase of cotton production. With an earlier blight, this could force the U.S. to be more export-oriented in its industrial production, rather than relying a growing internal market. However, American goods tended to be more expensive than their English counterparts, therefore wages would have be reduced to make them more competitive, and perhaps importing more low-cost labourers would be necessary.
The effects on slavery would be important too, as cotton was labour intensive and prior to the Civil War slaves had become Virginia's largest export to the cotton-producing states. Many large plantation owners could go bankrupt, forcing them to sell slaves at reduced prices or simply free them. There would have be a shift to less labour intensive crops in the cotton belt, especially maize (corn). There could be a greater push to "repatriate" freed slaves to Africa as well, though I imagine due to logistical problems this would perhaps be limited, with fewer than 50,000 being repatriated. Though this would have a huge impact on Liberia.
It could also trigger more Americans to simply abandon the South and move west, perhaps causing more territorial expansion at the expense of Mexico. Keep in mind that cotton growing in the Far West managed to escape the boll weevil. Sonora, Sinaloa and Baja California are the largest cotton growing regions in Mexico and were sparsely populated, so perhaps a land grab for these regions would occur. However, I imagine many farmers, especially the smaller ones will avoid planting cotton. Many can become grain farmers, especially with the rising demand for wheat in Europe, others can go into ranching. Politically, an exodus of people from Georgia, South Carolina, Alabama, Mississippi, Louisiana and Arkansas to the west would reduce the weight of these states in Congress.
Internationally, the British textile mills were dependent on American cotton and this could force more cultivation elsewhere, such as India, Turkey, the West Indies, Brazil. In the short term it will push up cotton prices, hurting British industry until it adjusts to new types of cotton.