One of the many reasons why Blighttown is amazing is because of all that you went through before.
During the first hours of the game, you never move too far from Firelink Shrine, in big loops connected by shortcuts, and you get to familiarize yourself with the environment. You slowly build a mental map and the environment becomes easier to navigate. Backtracking is always an option, made easier with the shortcuts, and you are never lost for long. The passages in dark indoors are always brief and you find many vantage points that let you orient yourself and build the mental map. You travel up and down at times, but mostly horizontally in circles.
Then after the Capra Demon, it is the exact opposite. The kitchens then the sewers are always dark, cramped, with almost no shortcuts. You lose any sense of orientation, the only certainty is that you came from upward. Backtracking becomes less and less an option the deeper you go. You are now forced to only go down, always further from Firelink Shrine.
Then when you finally find a huge room with daylight visible from the ruined ceiling, you meet the Gaping Dragon, the biggest and scariest monster yet, giving you a sense that you will find more than giant rats at the bottom of it all.
At that point you are really fatigued of these dark and disgusting environments. You just want to be at Firelink Shrine again, feeling that you have some choice in where to go.
Then you enter Blighttown, in a huge tunnel even darker that what came before. After the rats and slime above, you find there humanoids again, that built scaffolding everywhere in the sewers. There is another civilization down here, different from all the previous hollows that were inhabitants of the castle. Then finally you end up on the big stone arches, in this huge area, a majestic sight despite all the ugliness surrounding you. After more efforts you descend at the bottom of Blighttown, in a swamp. At last you regain some sense of freedom, you can chose in which direction to go of this huge swamp, not contrained in your path by the scaffolding anymore. Free but what feels like irremediably cut off from Firelink Shrine and the world above. Then you explore this new world that you arrived in, not sure if you actually chose to come here. You build a new mental map, that connects somehow with the first one by an interminable and thin link. You are free and yet trapped down here. The castle ate you in its kitchens, digested you in its internal labyrinth, through the sewers, then finally spew you in Blighttown.
The genius of Blighttown seeps through other parts of the game. Anor Londo is even more beautiful and stricking because of all the ugliness you found deep down. Ash Lake feels like a place holy and pure because it was preserved from all the filth above.
Reading in other posts that others love Blighttown as much as I do makes me happy. It is the best part of what will always be one of my favourite games.
I love this painting of Elric of Melniboné by Michael Whelan for several reasons. One of them will be obvious to you.