I'd like to say the idea of Hekate being a crone is a modern idea. She is simply a maiden in Greek myth. Her triple goddess form has to do with the crossroads, as well, and not the "maiden, mother, crone" ideal that is projected on her in modern lore.
Her symbols are dogs, the torch or two torches, keys, snakes, the underworld, owls, bats, crossroads, polecats, frogs, -- to name a few. (Basically, animals associated with witches. This symbolism for her is especially prominent in medieval art.) She is also rather associated with ghosts and evil spirits. She tends to send those on people while also protecting them from them.http://www.theoi.com/Khthonios/Hekate.html
As for the darkness ideal, she is a rather mysterious and dark goddess. Her devotees are wrapped in old stories of being witches, her cult being filled with them. Witches were to be feared back then. It is also said, that these witches favored the superior position of women on top during sex which was a big deal to patriarchal Greece. (She is likewise compared to Lilith later on.)
I think she was a foreign goddess from Asia minor who became Hellenized. To me, though on the darker spectrum, Hekate is about balance. Hence how she can protect people from evil whitlist send evil spirits on them. (She tends to have a few monster daughters too, but their names escape me atm.) There is a lot to be said of how she lights the way, I think it's important to remember that she is often the light in the darkness.