This is a pretty vague question so I will do my best to answer all possible perspectives.

One thing you should know; however, is that Bless's story is based around a war between two relatively powerful factions. In many places the landscape reflects this war. Additionally, each race has a different starting area that is reflective of their lore. Sylvan elves start in a relatively bright forest, while the Mascu begin their journey near a large steampunk city. Despite these different initial backgrounds, the storyline eventually converges and you will be seeing the same environments (regardless of your faction choice) the closer you are to the center of the continent the more war-torn your environment tends to become, naturally there are some exceptions to this.

From a lore perspective, none of the 7 launch races have an exceptionally dark backstory or starting area.

• The Mascu are a race foreign to the continent who find themselves in the middle of the war after a mechanical failure leaves them stranded.
  
• The Amistad (Union) are a humanoid race that were once followers of the Northern Dominion, they eventually fled their homes to escape what they viewed as an opressive system to instead pursuit their own beliefs.
  
• The Aqua-Elves (Union) are an elven race that were once part of the Elven Kingdom, they are decendents of a group of exiles that live on an island which gives them their name (aqua) they are a proud race that values individualism and progress (both social and scientific)
  
• The Pantera (Union) are a nomadic bestial race who roam the southern half of the continent, the joined the Union in an attempt to receive recognition
  
• The Habichts (Heiron) are a humanoid race that values tradition, loyalty, and courage. They aspire to lead the Heiron's conquest of the southern races in an attempt to unite the world under one banner.
  
• The Sylvan Elves (Heiron) are an elven race that values tradition, knowledge, and magic. They have entered an alliance with the Habichts and the Lupus
  
• The Lupus (Heiron) are a bestial race that joined the Heiron in an attempt to bring peace to the world
  

Upon examination it becomes clear that this war is being fought over a difference in ideals, and as every race has faced a different type of struggle in their lore it is up to you to ultimately decide which is the darkest for you.

Art-wise the elven races and the Mascu have relatively bright starting zones, the Habichts, Amistad, Pantera, and Lupus all have slightly darker starting locations, I wouldn't say one is darker than another.

While I don't know why this is important to you, much of what makes a certain faction dark is up to interpretation in this game. No single race began the war, is it the Habichts fault for valuing tradition over the worth of an individual? Or did the group that eventually became the Amistad go to far? Were the Sylvan elves just in their exile of the Aqua elves, did the Aqua elves do something truly egregious, or were they simply following their own beliefs as individuals? These questions are (almost) all left for player interpretation and there is no official answer from the developers. This allows players to chose who they think is right and fight for them (that or just choose whatever looks coolest to them and live with minimal regrets)

If I had to break it down I would do so this way

• Races of the Heiron faction value tradition, honor, loyalty and courage. They are a traditional empire and seek to expand their rule.
  
• Races of the Union faction value independence, progress, and integrity. The are a loose alliance trying to prove their power and sovereignty.
  
• The elven races have brighter physical designs than the other races
  
• The Mascu are oddballs and found themselves in a foreign, war-torn land after their ship was destroyed.
  
• The bestial races are more rugged than the other races
  

Again, no faction is darker than the other. The world you explore is just as damaged no matter who you side with, each faction is guilty of committing their own crimes (as is each individual race) and each race is fighting in this war for their own reason. Additionally, each race has a counterpart in the opposing faction, so one doesn't even have more monstrous races than the other. It all seems to be open to interpretation.