These ladies are mirror images of one another, in more than one way actually. DC's Phantom Lady was the premier heroine of her original publisher, Quality Comics, during the 1940's. Bestowed with a black-light beam projector given to her by a friendly professor, debutante Sandra Knight donned this guise (such as it was at the time).
The mysterious Darkling, known through unverified sources as Darla Lang, was a hero in her own right. Although she didn't project dark like her name would indicate, her true ability matched up well to that of Phantom Lady's later legacy, dimension transportation. Both caped lasses joined predominantly masculine teams to save the Multiverse from various threats, and neither returning to her home world because of their self-sacrificing dedication the protection of innocents throughout the cosmos.