There are different levels of alkalization so you will find alkalized cocoa powders in different shades of brown. The kind used in Oreos is ultra-dutched and are blacker in color and has a pH of 8.
On the other hand, natural cocoa have fruity notes and is acidic. It doesn’t keep as long as the Dutch-processed variety, it’s more expensive and less accessible. I only know of two brands – Hershey’s and Goya while there are dozens of Alkalized Cocoa Powder options out there.
The acidity of the cocoa as well as the vinegar and milk that I used for this recipe is balanced by the baking soda, an alkali. Here, we can understand that neutral pH is a goal in making a recipe. This allows for optimum rising of the baked product since the interaction of acids and bases will produce Carbon Dioxide (air). So if you encounter recipes with baking soda and cocoa powder, make sure to use the natural kind. I have observed that it is not common practice to write whether to use natural or alkalized cocoa powder in recipes.
Head over to Serious Eats for a more extensive scientific explanation.