Cocoa and Chocolate.
Cocoa powder, or cocoa solids, are just the low fat component of chocolate. The other being cocoa butter – a pale-yellow vegetable fat.
Make sure you buy your cocoa powder without any added sugar or other trappings. The one I get is 99% pure cocoa and makes wonderful hot chocolate!
Chocolate comes in many forms. For smoothies you want to use the purest one you can get. However, if you get it too pure – then it won’t blend without a little effort. For that reason I tend to buy cocoa powder – as it’s just easier to use and lower in fat.
In their purest forms, i.e. without added sugar etc, both cocoa and chocolate are not intrinsically unhealthy. Also, normally you just use a teaspoon or two so they won’t add too many calories either.