"They Met in the Dark" is like the first draft of a Hitchcock film, before the better plotting, interesting camera-work, and Hitchcock wit is added. In fact, it's a blend of "The 39 Steps" and "The Lady Vanishes." With many of the same elements:
Man and woman meet during a mysterious incident. Check. They are forced to stay on the run together. Check. The "McGuffin" is a secret message about the military. Check. The secret is conveyed by a music hall entertainer. Check. The unlikely couple end up in love. Check.
It's interesting to see the difference between a perfectly fine movie and a great one. Hitchcock created striking lighting effects, innovative camera moves, and darker, more menacing threat.
"They Met in the Dark" is a perfectly charming diversion and a nice, little movie. But pales in comparison to the Hitchcock films of the same era.