Here the words "us" and "we" refer to Elihu, Eliphaz, Bildad, and Zophar, but not to Job. Elihu uses this phrase sarcastically. (See: [[rc://en/ta/man/jit/figs-exclusive]])
Elihu speaks of the inability to understand as if it were having darkness in one's mind. Alternate translation: "because we do not understand" (See: [[rc://en/ta/man/jit/figs-metaphor]])