"But literally, there are none more black."
No, not the resource. The music.
Just like rap and emo, people's perception of metal is completely destroyed by stereotypes, misleading mainstream groups, and an ugly past of fame. Metal, as a genre, is capable of much more than simply chugging its way out of speakers on some stoner's vintage car. When people talk about metal, people usually come to one of three possible reference points.
1.) 1980s era glam-metal that cared more about image and excess than actual music.
2.) Sludgy, beefy drop-d metalheads, cheifly inspired by 1970s classic rock.
3.) 1990s era industrial metal revivalists, popularizing the term numetal.
Examples #1 and #3 are examples of the music industry latching onto a niche and exploiting it for all it's worth. There are certainly good bands that came out of the glam-metal phase (Guns 'N Roses) and good bands that were born in the numetal era (Deftones, Rage Against The Machine). But overall, both genres are generic exaggerations of what metal is supposed to be (is anyone familiar with Twisted Sister or Limp Bizkit?).
Example #2 is probably the most respectable of the three, but is oft the most misunderstood of the three. Their fanbase is just a bunch of dudes in their 30s wearing flannel shirts with long hair. That sludgy, beefy sound, however is the groundwork that all metal is laid upon. Metal was arguably invented around the time of Black Sabbath, and has proven to be one of the most enduring subgenres of rock & roll (punk rock has endured with more strength through its pop components and adaptablility).
Metal is more about an epic sound than anything. Metal is bigger than rock, which is why many people cite it as sounding too melodramatic. By nature, metal must be over the top, because it is a genre based on swelling, loud sounds. From the crushing tech-metal of Dillinger Escape Plan and Between The Buried & Me, to the expansive drones of Isis, Pelican, and Mastadon, metal is a genre based on a struggle of proportions larger than the topics of most modern pop songs.
This is why metal -- and I mean metal -- has never truly broken through onto the pop charts without sacrificing its core values. It doesn't have to deal with satanism and evil - in fact, the best metal never touches on such arcane topics. The best metal is of personal struggle, whether it be from the point of view of an outcast, the oppressed, or the forgotten. This idea has been neglected so often as to make it an ugly stereotype; that the guy in the metal band is a depressed psychomaniac that drinks blood. It's just a point of view, presenting itself in an epic, sonic form.
For further reference points, here are some sites streaming the music of modern metal bands:
Dillinger Escape Plan
Between the Buried & Me