Nintendo, first seen using this method for differentiating game ratings with mature video games for the Wii (in Japan, anyway), will place a black banner, instead of a white one, for Nintendo 3DS games when applicable.
On the Wii, the black box art template was used for all games that were a teen rating or above. The first game seen with the black banner on the 3DS is Ubisoft’s Splinter Cell 3D.
Although this information may seem arbitrary to some of you gamers, this is actually a smart move by Nintendo. Many parents complain about their children uying adult games, and then going on to claim that they did not know, in spite of the large rating in the corner of the cases. Now, there is a large indicator of the rating level of a video game, all thanks to the upcoming black banner. This will make it easier to not only spot games for the hardcore gamers, but it will allow parents to monitor their children’s library much easier.
Source: Nintendo Also Packages “Mature” 3DS Games in Black Boxes Too [Silconera]