Ben is Willy's adventurous and lucky older brother. Of course, he's dead, so he only appears in the play as a character in Willy's troubled imagination. Willy totally idolizes Ben because he was an adventurer who escaped the world of business and got rich quick by finding diamonds in the African jungle.
One of Willy's lifelong regrets is that he didn't go with his brother to Alaska. Unlike Willy, Ben was able to take a risk and stray from the world of fierce ambition and competition. Willy interprets Ben's good fortune as undeniable proof that his dreams of making it big are realistic.
Willy also associates Ben with knowledge and self-awareness, qualities that he himself is severely lacking. Willy always wants advice, and Ben gives it. Of course, it's frequently not very good advice and is usually the product of Willy's own imagination.
In his imagined conversations with his brother, Willy pries him for information about their father, about how he succeeded financially, and for advice about parenting Biff and Happy. It's hard to talk about Ben and his responses to these pleas, since he is either a memory of the past or a figment of Willy's imagination. And given Willy's complete lack of credibility, it's hard to tell even these apart.
But one thing we can take as true with reasonable confidence is the scene where Ben fights Biff. Ben wins, but only by cheating, informing the boy that that's the only way to win. There's some sketchiness surrounding his success in Africa (we're thinking he wasn't just handed the diamonds and sent along his way). He even says, in Willy's imaginings, "The jungle is dark but full of diamonds." That's big stuff right there.
Considering Ben's self-serving nature and amoral proclivities, the word "dark" connotes more than just shadows under the trees. We're not going so far as to say words like "evil" or "Darth Vader," but Ben's success is certainly blemished by his apparent use of cheating to get what he wants.