To understand why our tephillin are always painted black, we must first understand a little bit about the Torah Sh’Bektav and the Torah She’bal Peh, the written versus the oral Torah.  When Hashem taught Moshe Rabbeinu the entire Torah on Har Sinai, He taught him both the pesukim of the Torah Sh’Bektav as well as the accompanying explanations and halachot, which form the basis of the Torah She’bal Peh.  These various explanations and halachot flesh out the terse pesukim of the Torah She’Bektav and, in the case of many mitzvoth, actually provide the essential information that is necessary for us to even be able to perform the mitzvah, at all.  These explanations and halachot, which were handed down orally from generation to generation, provided the volume of material that Rabbi Yehuda HaNasi transcribed when he compiled the Mishnah which forms the foundation of the Torah Sth’bal Peh.
   
    The mitzvah of tephillin, more than most, clearly demonstrates for us the need for the Torah She’bal Peh.  The fact of the matter is that without it we really wouldn’t know how to go about performing the mitzvah of tephillin.  This is the case because the pertinent pesukim in the Torah only tell us that tephillin should be worn but not how they are to be constructed or how they should look.  When the Shulchan Aruch, the code of Jewish law, provides these details, such as the requirement that the rezuot, the tephillin straps, have to be black, it refers to it as a Halacha Moshe MiSinai.  The Rambam, in the introduction to his commentary on the Mishnah, explains that a Halacha Moshe MiSinai is a halacha that was passed down through the generations that could not have been learned from logic or from expounding the pesukim of the Torah. 
   
    Although a Halacha Moshe MiSinai, in a sense, is an arbitrary halacha that we don’t have any reason or basis for, it is taken very seriously and given the status of a Torah law.  Indeed, the Mishnah Berurah writes that if the straps are not black, one has not fulfilled the mitzvah of tephillin.  He writes that they should be as black as a raven and if they have become worn or peeled, so that the black has come off in places, one should be very careful to repaint them.  It is very common that the rezuot will wear down and chip at the edges in the place of the knot and the wrappings of the hand.  Today tephilin paint is available in small bottles that are similar to white-out and also in specially made magic markers.  One additional halacha, in this regard, is that the tephillin need to be painted with the deliberate intention to do so.  This means that one should say, “leshem kedushat tephillin” or “for the sake of the holiness of tephillin” before painting the tephillin.
   
    It’s very interesting to note that although the Gemara tells us that the halacha that the straps should be black is a Halacha Moshe MiSinai, it does not mention the color of the tephillin batim or tephillin boxes, themselves.  Indeed, there are halachic authorities that allow the boxes to be any color.  However, the Shulchan Aruch writes that it is a mitzvah to paint them black, as well. The commentators explain that painting the batim black is a form of beautifying the mitzvah and is, therefore, a mitzvah in and of itself.
   
    So, in summary, we see that tephillin are painted black because the Torah She’bal Peh tells us so.  The fact that tephillin have always been painted black, in this fashion, ever since Har Sinai, serves as a powerful reminder that the Oral Torah is just as essential to Yiddishkeit as the Written Torah.