Both words (as verbs) mean essentially the same thing, which is to coagulate or congeal.
With food, curdle means to coagulate and become sour, thus has a connotation of spoilage. For example if milk becomes curdled due to age or exposure to high tenperatures, it is generally considered spoiled and unusable. When milk is left at room temper, it seperates to form curds (lumpy white substance) and whey (clear watery liquid) which are either consumed as is or used to make cheese. Clotted cream is made from the cream of the milk and is generally considered tasty.
Blood can clot (verb form) or form clots (noun form). Curdled is not used to describe blood.