WordNet sense 1 (switch (a car's headlights) from a higher to a lower beam):

dip

WordNet sense 2 (become or make darker):

darken

WordNet sense 3 (offering little or no hope):

black, bleak

WordNet sense 4 (without hope because there seems to be no possibility of comfort or success):

hopeless

WordNet sense 5 (lacking clarity or distinctness):

faint, shadowy, vague, wispy

WordNet sense 6 (not clearly defined or easy to perceive or understand):

indistinct

WordNet sense 7 (slow to learn or understand; lacking intellectual acuity):

dense, dull, dumb, obtuse, slow

WordNet sense 8 (lacking or marked by lack of intellectual acuity):

stupid

WordNet sense 9 (make dim by comparison or conceal):

blind

WordNet sense 10 (become vague or indistinct):

blur, slur

WordNet sense 11 (made dim or less bright):

dimmed

WordNet sense 12 (lacking in light; not bright or harsh):

subdued

WordNet sense 13 (devoid of or deficient in light or brightness; shadowed or black):

dark