dunkel: b) unklar: in dunkler (ferner u. geheimnisvoller) Vorzeit;
eine dunkle (schwer deutbare) Textstelle; <subst.:> jmdn. [über etw.] im Dunklen (im Unklaren, Ungewissen) lassen; etw. liegt noch im Dunkeln (ist noch ungewiss).
This is the only definition in Duden of "dunkel" that I can think *might* be translated as "opaque" - you can make an opaque comment or an opaque speech, etc.
"In an opaque speech recently, the chancellor set out ground rules for the line between private and public." http://society.guardian.co.uk/comment/column/...
You would not, however, say "in the opaque past" but "in the dim and distant past". Would you say "eine dunkle Rede"? Doesn't sound right to me... i agree with Uho that some examples would be more helpful.
"Dark" as a synonym of "opaque" also only works in very specific cases: take a look at the other synonyms of "dark" and you'll see they often have extremely different meanings according to the situation: http://thesaurus.reference.com/search?q=dark
The business partner isn't exactly wrong, though - opaque, of plastic, means you can nearly see through it but not quite. The colour is irrelevant, mind you.