The phrase "bliss of solitude" appears in the final stanza of "I Wandered Lonely as a Cloud." Wordsworth writes that "They flash upon that inward eye/Which is the bliss of solitude." The "they" the line refers to are the golden daffodils alongside the lake that appear in the first stanza of the poem.
In his "Preface to Lyrical Ballads," Wordsworth discusses "emotion recollected in tranquillity." The daffodils represent the source of this type of emotion, as the poet recalls them as he lies alone upon his couch in a state of contemplation. The Romantics, including Wordsworth, believed that nature was the source of a great emotional response in individuals. By enjoying the "bliss of solitude," or the quiet contemplation that allows people to think about and understand their emotions, individuals can arrive at a state of transcendence. This is the "bliss" that Wordsworth refers to. Only by contemplating beauty and the sensations that nature produces in private can individuals have this type of transcendent experience.