I like to think the film was a con in itself

I like to think Stephen and Bang Bang are very much alive. I might be wrong, but then again, I'm not sure there is right or wrong when it comes to things like this, so I'm just gonna pop my theory.

"The day I con you is the day I die," says Stephen earlier in the movie. I think he conned Bloom about his death so Bloom could really go ahead and live a life with no strings attached to his ever so dominating brother.
Earlier in the movie, Stephen not only mentions that real blood turns brown after half an hour, he also mentions that they should go ahead and do something to improve their fake blood, so I think it is a plausible scenario that Stephen might have engineered a way to make better fake blood.
The truth is that even though Stephen's priority was to make his little brother happy, he probably didn't want to die, so I think it would give more sense to his "the perfect con is the one in which everyone leaves with what they wanted." Stephen and Bang Bang get the money, Bloom goes with the girl without feeling like he's living his brother behind.
Last but not least, in the final scene, the curtain covering the stage has the image of a sunset turning into night, which was one of Stephen's phrases, and I really don't think a thing like that could possibly be a coincidence.
I also think I saw a small glimmer of light in the door leading to the exit of the theater before Stephen 'dies'.

Even if it's not what the filmmakers intended, I like to think that this is how things turned out.