"Sombra Negra" is Spanish for "Black Shadow". In El Salvador Sombra Negra was one of several death squad groups. They are comprised of mostly police and military personnel, and target criminals and gang members, however their actions are not officially sanctioned, making them vigilantes. They were usually indiscriminate about who they targeted, innocent people were often killed by them. They often did not just outright kill their victims either, they were known to torture them for hours or even days, and they also were known to rape female prisoners.
When Efraim takes Daniel to the fountain, the on screen translation says "every Tuesday, at 5pm - a little miracle." In Spanish Efraim actually says "Every day, at 5pm -a little miracle". This makes the fact that the PorAgua security guards are there when the fountain comes on much more plausible.
The title of the episode is the same name as a previous show that Alycia Debnam-Carey worked on, The 100 (2014).
The bulk of this episode is almost entirely in Spanish.
This is the first episode in the series not to feature any members of the Clark family.