It's not surprising that a poem concerned with coming to terms with death and dealing with grief might employ lots of imagery relating to darkness and light. It's all about finding that proverbial light at the end of the tunnel, right?
Patterns of light and dark, sight and blindness are all over the place in this poem and make up some of the dominant imagery. Better pay attention anytime you see a reference to light, dark, shadows, eyes, seeing, or blindness.
Tennyson uses light to signify things like life (2826) and the hope of faith. Check out how this works right up front in lines 17-20:
Our little systems have their day;
They have their day and cease to be:
They are but broken lights of thee,
And thou, O Lord, art more than they
The speaker characterizes the puny "little systems" of humans (probably referring to human knowledge) as just "broken lights" of God. So, everything humans come up with—and keep in mind that during this time period science was very much on the radar and battling it out with religion—is just a pale reflection of the divine light.
Not surprisingly, the lightness imagery is also wrapped up in being able to see or not see, and being blind on a figurative level. So, a "haze of grief" (517) makes it hard for Tennyson to know whether or not he is actually seeing the past as it truly was, or if he's sugar-coating it. Similarly, where human perception can be clouded, divine perception is always crystal-clear. The eye in Canto 26 that sees everything suggests God's all-seeing and all-knowing sight that exists outside of time.
The flip side is also true. While light and seeing have positive connotations, shadows and darkness are usually the opposite. So, the house that Arthur once lived in is now a "dark house" (165). Also, anytime you see Shadow (and yes, that's with a big, bad, capital letter), Tennyson is usually talking about death personified, which has snatched away Arthur.
But when light and dark are blended, it's not always so clear-cut. For example, toward the middle of the poem Tennyson seems to make things a bit more complicated, mentioning "witch-elms" that make the lawn look "dusk and bright" (1782), and describing how Arthur once liked the shadows cast by the sycamore tree (1783-1784). As Tennyson's attitude toward his loss starts to change, the borders begin to blur a bit (wow...pardon our alliteration).
In fact, the "one mute Shadow" at line 612 is pretty ambiguous. It seems to mean both death and also Arthur's spirit, which is watching his still-living friends sing and play their games at Christmas-time.