The speaker describes the way that "Grave men" fight their impending death.
Notice the pun on "grave," which could either mean that the men are very serious, or that they are dying.
These serious dying guys realize that, even though they are weak and losing their faculty of sight, they can still use what strength they have to rage against death.
So, even though their eyes are going blind, these men can "see," metaphorically speaking, with an overwhelming certainty or "blinding sight," that they still have a lot of power over the way they die, even if not the timing.
Instead of getting snuffed like candles, they can "blaze like meteors" (line 14). They're planning to go out with a bang.