We know that it is not a good thing if your retzuous flip over to the other side. But what if they are the kind of retzuous that are black on both sides? Does this feature elimante the problem? I asked Reb Avraham Tzvi Wosner this morning and he told me that even when they are black on both sides, the outside is still nicer because it is shiny. The gemara says that the nicer side is supposed to be on the outside. Therefore, even if they are black on both sides one should still be careful to not let them flip over.