careless > abandoned, not caring; artless; unconscious, free from care blue > {Livid; low-spirited, affected with fear}
5 And dimmed sight, with pale and deadly hue,
deadly hue > deathly appearance
6 At last she up gan lift: with trembling cheer
gan > did cheer > expression, countenance
7 Her up he took, too simple and too true,
simple > (The knight and Duessa both; simple also has the meaning "free from duplicity"; she is then too simple and too true)
8 And oft her kissed. At length, all passed fear,
passed fear > [fear passed]
9 He set her on her steed, and forward forth did bear.