Excell 2010: How to set/reset the default color for cell borders

I apply cell borders often; the line color has always been black. Yesterday I selected a small range and changed the border around that specific range to a medium blue (could have been any color). Now, much to my dismay, that blue has become the default color for all borders in all workbooks. So far nothing I do in the Border tab of Format Cells, or anything else, reverts the default to black. There's no apparent way that I can find to set or reset the color. Can someone show me the secret? Thanks in advance.