Why the name "Dark"?

They combined two ideas they were working on and used the title of one of them.

In an interview with Jantje Friese and Baran bo Odar during the 2018 Berlinale, Baran bo Odar tells how the show came about. They were asked for a show by Netflix, after turning down Netflix' offer to turn their successful film Who Am I – Kein System ist sicher into a show.

"We were working on a time travel trilogy back then, but like really big entertaining action movie, and we worked couple years on a show that was called 'Dark', another show or movie that was called 'Dark' that was based on the real story of the serial killer Marc Dutroux, who was a serial killer in Belgium in the early nineties, which was really, really, really, really dark as a story."
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"We really combined these two ideas. We took the title out of that dark movie 'Dark', and said, let's combine family drama in a small town where kids disappear combined with time travel."

I haven't watched the show but I Googled for a description:

When two children go missing in a small German town, its sinful past is exposed with double lives and fractured relationships that exist amongst four families as they search for the kids...all of whom have a connection to the towns troubled history. The story includes supernatural elements.

These are typically thought of as dark elements in a story. And this is most likely why the writers called the programme 'Dark'. After all, writers choose words for their emotional resonance, especially in drama.