Safe to Use Obagi Nu-Derm That Turned Dark?

The dark color seems to suggest that the product has undergone oxidation.  While it is probably not dangerous to apply, I would not recommend it.
Good luck.

This product contains hydroquinone and Vitamin C. Vitamin C is an unstable compound and oxidizes readily. This most likely accounts for the brown discoloration. While I doubt whether it is harmful, it probably has lost some potency. This is a prescription product due to its containing 4% Hydroquinone. It would be a good idea to ask the physician who prescribed it for another bottle if it was received in that condition.
(By the way, we once carried Skinceuticals, many of whose products contained Vitamin C. Sometimes this would oxidize similarly and turn brown. The rep said that it kept its potency, but frankly I did not believe her.)