I noticed today that my stool was a bit darker brown than usual. definitely not black, just darker brown than it usually is. what caused this?

Most likely normal: Our stools can vary in color depending on what we eat. Some foods such as beets clearly do this, while the amount of fiber and protein can shift the color slightly. If you pass soft, well formed stools on a regular basis, don't feel pain with passing one, don't see any blood or black tar-like substance in or on it, then you most likely are fine.

Diet: Though blood can make your stool black, dark brown stool is not necessary worrisome. Dietary changes and medications can also cause changes in stool color.