Ironically, most of the light imagery in this poem is more about the lack of light than the presence of it, but one cannot exist without the other. Rossetti uses light imagery in a totally traditional way in "Up-Hill," mainly as a symbolic representation of good and evil. The bright side (pun intended) to the presence of so much darkness in the poem is the inn on the hilltop that's waiting at the end.
Lines 3-4: Day and night are common ways in which light and darkness are depicted, and "Up-Hill" is no exception. By saying the journey will take "from morn to night," Speaker #2 is telling Speaker #1 that the trip up will not be easy. From a spiritual perspective, it's implying that all those seeking salvation are bound to slip up and sin occasionally.
Lines 5-6: By claiming that the roof is there "for when the slow dark hours begin," Speaker #2 frames those dark hours as something Speaker #1 needs protection from. This could be a reference to the literal dangers of being outside on a mountain at night (animals, hoodlums, and the like), or the spiritual dangers of evil temptations sent to cause people to stray from God's light into the Satan's wicked darkness.
Lines 7-8: Literally thousands of people assume that, when Rossetti says the darkness cannot hide the inn, it inevitably means it's lit up in some way. This is a very logical conclusion to reach, but it's interesting that the poem doesn't actually say that the inn is lit up. One thing that is there, however, is the parallel between Rossetti's inn on the hilltop and the "city upon a hill" discussed in the Book of Matthew (and lots of other places, too—the religious image appeared in speeches by Presidents Ronald Reagan and John F. Kennedy, just to name a few).