At first I thought you must be referring to Word 2003, but no, you do mean Word 2013.

I know of no update to Office that did this, by itself. I think it must be an interaction with your Windows settings. I know of no way to change this within Office. It is possible that within Windows you changed the display font from default to black. (This is just a guess. Windows is a mystery to me. You may be able to get more information on a Windows forum, though.)