With poetry, there is always many explanations for why an author pens what he does and the meaning thereof, but I found this explanation on line and thought I would share with you. Knowing Blake as a poet, I think maybe his intent was layered, partially to talk about the rungs of society and then again to make the poem sexual in nature, about man and woman and our lot in life. I hope this provides you with some material to ponder.
( a poet as well as a Wyzant instructor)
"The Blossom" is full of cheerful images of life, such as the "leaves so green", and "happy blossom". The poem tells the tale of two different birds - a sparrow and a robin. The former is clearly content with its existence; whereas the latter is distraught with it, meaning the second stanza becomes full of negative, depressing images. This could be an attempt by Blake to portray the opinions of different groups of society - with one class (assumedly the ruling classes) content with maintaining the Status Quo, and the other class unfair with the changes required - as Robins traditionally appear during the Winter, one could assume that it is upset at having the miss the exciting, lively critiques that occur with summer - such as Blossoms.
Another possible interpretation is a sexual one, where the poem represents the joy that can be found through innocent sexual love. The sparrow, seeking his cradle 'swift as an arrow' has been interpreted in a phallic sense, and demonstrates the innocence and joy of free love. The 'happy blossom' in this sense is therefore the female sexual organs, which is happy upon seeing the arrival of the sparrow.
The 'sobbing, sobbing' robin has been interpreted in several different ways. Either it is the opposite to the sparrow's open love, a creature who has been harmed through love or possibly violated, or it is another creature rejoicing in the joy of sexual love, in which case its sobbing could be orgasmic. Whilst this does sound a rather strange interpretation for a poem in 'The Songs of Innocence', Blake himself believed greatly in the joys of free love. Many of his poems, such as 'The Little Girl Lost and Found', 'The Lilly' or 'The Angel' also follow this theme of giving in to desires and sexual love.