In the fourth canto of Don Juan, stanza CX., Byron says:
Oh, darkly, deeply, beautifully blue,
As some one somewhere sings about the sky.
Byron was mistaken in thinking his quotation referred to the sky. The line is in Southey’s Madoc, canto V., and describes fish. A note intimates that dolphins are meant.
“Though in blue ocean seen,
Blue, darkly, deeply, beautifully blue,
In all its rich variety of shades,
Suffused with glowing gold.”