One of the reasons why hyperlinks are off different color than your text is so that your visitors/readers may know that there's more information they can read. It's also a good way of sending traffic, and even
PR juice, from the index page to your inner pages
One thing I'm sure about is black hat optimizers loves to match the color of their hyperlinks with normal text to avoid loosing traffic. Because when people see that there's more to what they reading, they'll eventually click the outgoing link. Another thing is, back hat SEOs also do this so they can pass
PR juice from their index page to some of their inner pages. They do so by placing a hyperlink on a keyword from the index page, which isn't related to the article/content visitors are reading, and then change the color.