Supplementing Graham's response:

Even if you were to modify the Ribbon you cannot change the Word setting which is to default to Red. I suspect that this is because the meaning of the control (change font color) is more intuitive with red than it would be with black.

Again, it will be set at whatever color was last used during the Word session.
It resets whenever you restart Word. It has been like this since at least Word 97.

You could set a character style to be black or automatic if you wanted to do so. However, automatic is the default setting for most styles.

Understanding Styles in Microsoft Word
Tips for Understanding Styles in Word by Shauna Kelly (not as long but very useful)

Basic Concepts of Microsoft Word - from Shauna Kelly