Thomas Hardy's poem "The Darkling Thrush" describes a very bleak atmosphere where the speaker, Hardy, uses very dark imagery to paint a picture for the reader.
The words used to describe the setting epitomize a hopeless scene--spectre-grey, dregs, desolate, broken, haunted, corpse, cloudy, and death. Here, the words chosen instill in the reader the hopelessness which the setting holds.
It is not until the third stanza that "hope" enters the picture.
At once a voice arose among
The bleak twigs overhead
In a full-hearted evensong
Of joy illimited.
The thrush, unaffected by the hopeless surroundings, begins to sing. The speaker cannot understand how such a beautiful song can be sung or exist in a place like the one the speaker is immersed in. In the final stanza, the thrush is described as possessing a blessed Hope. The speaker, recognizing the hope shown by the thrush, fails to recognize why the hope exists.
Therefore, the blessed hope shown by the bird refers to the fact that there is always a silver lining in even the darkest of atmospheres. The blessed aspect refers to the fact that one must have faith in something greater. The allusion here, which comes from the word blessed, could illustrate the hope given by God, which is why it is blessed. What this instills in the reader is the fact that, even in the darkest times, God can provide the hope which allows one to see through darkness in even the most hopeless places.