This article argues that societies that ate roots/tubers (potatoes, manioc, yams, etc.) got as many nutrients as societies that ate grains, but had fewer reasons to develop complex societies (because grains can be stored in large quantities and are easily stolen). Hence, potatoes and other tubers were a "curse" to societies like the Caribbean Taino that relied on them for food.
I question this language of "blessing" and "curse." The assumption seems to be (if the theory is right) that developing a complex, hierarchical society is a good thing. Certainly it has led to many good things--and to many evil things.
It might be more accurate to say that the curse of tuber-eating societies is contact with grain-eating societies. The Taino weren't cursed by their manioc. They were cursed by the Spaniards.