The English theologian and historian Thomas Fuller appears to be the first person to commit the notion that 'the darkest hour is just before the dawn' to print. His religious travelogue A Pisgah-Sight Of Palestine And The Confines Thereof, 1650, contains this view:
It is always darkest just before the Day dawneth.
The source of the proverb isn't known. It may be Fuller himself, or he may have been recording a piece of folk wisdom. In 1858, much later than Fuller of course, Samuel Lover attributed the notion to the Irish, in Songs and Ballads:
There is a beautiful saying amongst the Irish peasantry to inspire hope under adverse circumstances:- "Remember," they say, "that the darkest hour of all is the hour before day."
It may not be the darkest time, however, it is the last hour of the night's darkness as well as there being a large contrast with the daybreak.
Of course some have skewed the saying to read: "It's always darkest before the dawn, so that's the best time to steal your neighbor's newspaper."