Maduro is the Spanish term for ripe; meaning fully developed.  A maduro cigar is characterized by its dark wrapper, typically black or close to it. 

In its most limited sense, the term maduro can refer to the color of a wrapper leaf, where the darker shade is oscuro.  A leaf can end up darker if it comes from a late priming or a stalk-cut plant which is intentionally harvested to produce darker leaves: see broadleaf, Brazil, and Mexico.  

A true maduro cigar will have a wrapper which has undergone a longer and/or more intense (higher temperature) fermentation.  The specific times and temperatures will vary from one cigar maker to another.  This 'enhanced' fermentation releases the natural sugars of the leaf, which produces a “sweeter” flavor.  In addition, the extra fermentation will round out the spices in the leaf and make it richer in flavor.

Until recently, maduros have not been produced in Cuba.  The extra care taken to transform the leaf produces a style of cigar that is popular in the United States.  Usually, maduro cigars cost more due to the extra time and care needed to create them.  Some cigar makers may try to speed up the fermentation process by adding heat, extra moisture, or pressure to the tobacco.   In addition, less honorable cigar manufacturers will artificially color their leaves with dyes to create a visual appeal.   Obviously this is disingenuous since premium cigar smokers expect a natural product produced through traditional methods.   Fermentation is a natural process which should not be expedited.  Any method of speeding up production may produce an attractive dark cigar, but it will not have the flavor profile of a legitimate maduro.  Ultimately, the darkest, blackest maduro will probably not be the best.