The title of this poem is like a warning sign. "The Black Heralds" comes from line 8. They're those powerful blows in life that are sent to us by Death. You know, no biggie.
Some people think that the black heralds refer to the four horsemen of the apocalypse, which are biblical figures that ride out to bring the apocalypse onto the world. (No, not the zombie apocalypse. The regular one.)
The significance of the title is that these heralds are sent to us by Death, which in a religious sense was brought about by the original sin of Adam and Eve. These heralds are a metaphor for those blows in life that get us down: violence, suffering, injustice, and disappointment. If they're sent to us by death, and they are messengers, bringing down the last judgment on us, then they are pretty bad mammer-jammers.
The title gives us a warning that the poem is going to be pretty dark—a black, or dark, messenger probably isn't bringing good news. (BTW, this is not a racial remark, but rather the idea of black, dark forces versus good. Still, it's worth thinking about the implications either way.) The title is also a warning that the poem probably has a religious theme, if it makes us think of the four horsemen.