Because spinach is fairly high in iron, it has been known to cause black stool in certain people who consume enough of it. Many people confuse black stool for stool with blood in it, but the fact is that when there is blood in the stool it look like tar and not simply dark. Those who find that they have experienced a change in urine color probably have not been drinking an adequate amount of water.
It is important to get lots of water in your body for a number of reasons, so make sure you do so on a regular basis. If you have been eating spinach and you find that your stool is a slightly darker color, it is probably nothing to worry about. Those who do find blood in their stool though will want to make sure that they seek medical attention as soon as possible because it could indicate a serious medical problem.