"Richly bleak": that sounds like an oxymoron, doesn't it?
No, no, we're not calling you a moron. We'd never say that. But we are saying that the narrator's attitude toward the townspeople and their reception of the winged man combines beautiful descriptions with sad undertones. García Márquez uses rich adjectives and figurative language to describe really bleak conditions. This quote really sums it up:
The world had been sad since Tuesday. Sea and sky were a single ash-gray thing and the sands of the beach, which on March nights glimmered like powdered light, had become a stew of mud and rotten shellfish. (1)
What we've got here is a really awful situation. The whole world has been sad for three days and everything is gray and rotten. But the narrator remembers how beautiful things could be, taking us back to March, when the beach glimmered beautifully. All through the story, the tone swings back and forth between bleak and rich, with the narrator constantly finding beauty in the hideous.
Sound familiar? This study in contrasts is one of the story's major themes. Check out "Symbols: Light and Dirt" for even more.