Both cocoa powder types are made from pressed cocoa beans to which their natural cocoa butter content has been partially removed to only leave a dry paste, which is then pulverized to make cocoa powder.
If the beans haven’t been treated with an alkali, the cocoa powder resulting is then called “Natural”. Natural cocoa powder (burgundy tin) is light in colour and has a pronounced chocolate flavour enhanced by the natural acidity of the cacao beans. Natural cocoa powder can be used in smoothies and many recipes. It is best suited for recipes that use baking soda*, to create a leavening reaction.
Dutch-processed cocoa powder (blue tin) is treated with an alkali (1–3% potassium carbonate) to neutralize the natural acidity of the cocoa beans. This neutralization process is carried out by treating the cocoa beans with a solution of water and alkali, first heating and then drying the cacao beans back to the proper working moisture.
This process gives the cocoa powder a more mellow taste, makes the cocoa powder easier to dissolve in liquids, and makes the colour richer. It is also ideal for topping cakes and coating truffles for example.
As for baking purposes, since alkalized cocoa isn’t acidic, it doesn’t react with alkaline leavening agents like baking soda to produce carbon dioxide. That’s why recipes that use Dutch-process cocoa are usually leavened by baking powder*, which has a neutral PH. While a recipe that uses natural cocoa powder will use baking soda as a leavening agent.
As mentioned, Dutch-processed cocoa powder is also ideal when making hot beverages because of its increased dissolving property as a result of the alkali treatment.
*as described on the packaging