http://fuckingblocksyntax.com/

As a local variable:
returnType (^blockName)(parameterTypes) = ^returnType(parameters) {...};

As a property:
@property (nonatomic, copy, nullability) returnType (^blockName)(parameterTypes);

As a method parameter:

• (void)someMethodThatTakesABlock:(returnType (^nullability)(parameterTypes))blockName;

As an argument to a method call:
[someObject someMethodThatTakesABlock:^returnType (parameters) {...}];

As a typedef:
typedef returnType (^TypeName)(parameterTypes);TypeName blockName = ^returnType(parameters) {...};