Of course she's supposed to be black. Just look at how big her lips are.
No, seriously, for some reason Japanese people have this stereotype that black people have enormous lips. I don't really get it but you see it all the time there.
...that's because of the U.S. >.> History lesson time! You see a vast majority of anime conventions come from U.S. animation conventions that were prevalent in U.S. animation during the U.S. occupation of Japan. In many ways you can say that the "anime style" of big expressive eyes originates from the Japanese copying Disney cartoons as that is the style of animation that Japanese animators were exposed to before they started working on their own homegrown industry.
Now then, if you go back to U.S. cartoons from that era (well assuming you can find any), you'll notice that blacks were depicted with giant lips in those cartoons. This was playing on stereotypes that were heavily played up in minstrel shows/blackface. Those depictions are now see as being horribly insensitive in the U.S., but Japan's population lacks a significant black presence, so they don't recognize the history for what we imported to them. >.>