The reformulation of Ungaro III is quite different from the original. The original was dark, with pronounced leather notes in the base. The newer version is not so dark.
Some ingredients can give fragrances a dark character; some combinations (especially of base notes) can do the same. Many people will say that a perfume in which the base notes clearly dominate is dark, especially with chypres and leathers, less so with fougères and florals. It is a little subjective, but one or more of the factors I mention will ususally get some people to call such fragrances "dark."