c. 1400, "dark," figuratively "morally unenlightened; gloomy," from Old French obscur, oscur "dark, clouded, gloomy; dim, not clear" (12c.) and directly from Latin obscurus "dark, dusky, shady," figuratively "unknown; unintelligible; hard to discern; from insignificant ancestors," from ob "over" (see ob-) + -scurus "covered," from PIE root *(s)keu- "to cover, conceal." Related: Obscurely.
The figurative sense of "not readily understood, not manifest to the mind or understanding" is from early 15c.; of persons, "not illustrious or noted, unknown to fame," 1540s. The more literal sense of "indistinct, without clearness of form or outline, hardly perceptible, not capable of being clearly seen through lack of light" is attested in English from 1590s.
In regard to the meaning of something said or written, obscure is general, being founded upon the figure of light which is insufficient to enable one to see with any clearness; this figure is still felt in all the uses of the word. [Century Dictionary, 1895]