Why did my green tea muffins turn black?
Your first step would be to stick to the recipe. pH changes are well known to interfere with plant pigments. Adding baking soda and vinegar can very well have interfered with something in the muffins, be it the tea or something else. I am not 100% sure this must have been it, but it is the likeliest reason. 

Also, I would recommend against making changes to a recipe before you have tried it as-written. You cannot know how changes affect it, and what unintended consequences they have, until you have established a reference. For example, you cannot know if your addition of baking soda made the muffins both fluffier and darker, or if it only made them dark without any change in fluffiness.