The gemara[1] states that there is a halacha l’Moshe miSinai[2] that retzuot must be black. It then cites a source stating that if the batim[3] are not black, the retzuot should be the same color, which contradicts the first statement’s implication that the retzuot are always black. The gemara explains that the outside of the retzuot must always be black; the other possible color is on the retzuot’s underside.
The Rambam[4] rules that the underside of the retzuot may be any color (other than red, which would be degrading to the tefillin if the retzuot turn over). He further writes that the underside of the retzuot should be the same color as the batim, and it is nicest if the batim and both sides of the retzuot are black. There is a machloket[5] regarding whether according to the Rambam, the retzuot’s underside must also be black when the batim are black, or if in that situation, it is merely preferable for them to be black. It is clear, however, that the Rambam certainly at least prefers, given that our batim are always black, that the retzuot be black even on the underside. Nevertheless, the Shulchan Aruch,[6] Rama,[7] and Mishna Berura[8] have no qualms with the widespread minhag that the underside of the retzuot is not black.
Some Acharonim cite the Arizal as positing that there are important kabbalistic grounds for the retzuot to be black on both sides.[9] Since we do not deal with kabbalistic issues that have not been integrated into common practice, we have nothing to add in this regard.
There is lack of clarity regarding two other possible advantages to the retzuot being black on both sides. First, it is considered very negative for the parts of the retzuot that fasten down the tefillin to turn over, and it is preferable that no part of the retzuot do so.[10] It is possible that the problem would be solved or at least minimized if the bottom side were also black, such that even if it were to accidentally face up, it is at least black. However, it is unclear if and to what extent this would solve the problem.[11] Second, some note that the way retzuot are blackened on both sides is by soaking them in dye through and through. This might be advantageous in that even if the outer layer of black paint peels or cracks in a way that could ordinarily affect the retzuot’s color and therefore their validity, the general blackness under that outer layer might save the situation. On the other hand, it is difficult to guarantee that the leather under the crack will be sufficiently black halachically, and relying on the assumption that it will be could be a mistake.
Weighing the above, there seems to be more advantage in the new practice of blackening both sides than disadvantage.[12] Despite this, my retzuot are un-died on the bottom, and I and many others, including prominent rabbis,[13] have no plans to replace them anytime soon. This approach of retaining our arguably less-desirable retzuot rather than switching to presumably preferable ones that are now available is based on the power of minhag.
Let us examine what the reliance on minhag means in this context. On one level, if there is a longstanding prevalent practice, especially one that is supported as perfectly fine in the most accepted sources, many will view this is an omen from Above that we need not try to improve on it.[14] In addition, there are several related issues that can make the change problematic, specifically because it is a change made with the claim or hope of improvement. One such issue is that of yohara (holier-than-thou haughtiness), another is that of casting aspersions on others in the past or present, and yet another is causing machloket. These general issues have many classical sources to which we cannot presently do justice, but they certainly are accepted as significant in certain contexts in the Shulchan Aruch.[15] Therefore, we discourage people from trendsetting in this matter, especially because the advantages we have mentioned are merely preferences and are not halachically compelling.[16]
Veteran sofrei stam can attest to a mushrooming of hiddurim/chumrot for standard upper-echelon tefillin over the last 50 years.[17] Many youngsters have more mehudar tefillin than their fathers and rabbis, and none of the mentioned problematic issues have resulted. However, most of these new hiddurim/chumrot are not readily apparent to others. Your question relates to the coloring of the retzuot, which is noticeable, and that noticeability of the stringency/hiddur could potentially create a real problem.[18]
Our recommendation is that you buy for your son that which is normal for his peer group. Do not be one who spreads a new practice, due to the above reservations. On the other hand, to the extent that the practice has already spread in your surroundings, it is not your doing, and there is little reason to be concerned that your son joining the trend will be a significant contributing factor in any negative element. Every generation brings changes, and, in retrospect, many of them are fine. Your young son need not be a minhag-preservation purist and should not feel that his tefillin are sub-standard among his peers.