That is coming directly from the show creator David Chase. He has FINALLY answered the question

In a recent interview with VOX, David Chase revealed the answer during a conversation in a coffee shop

Here is the way the conversation went:

I had been talking with Chase for a few years when I finally asked him whether Tony was dead. We were in a tiny coffee shop, when, in the middle of a low-key chat about a writing problem I was having, I popped the question. Chase startled me by turning toward me and saying with sudden, explosive anger, "Why are we talking about this?" I answered, "I'm just curious." He shook his head “no.” And he said simply, “No he isn’t.” That was all.

 

Read the entire article here. David Chase also says that the "cut to black" was based on a poem by Edgar Allen Poe called "Dream Within a Dream"

Here is the final scene from the TV show The Sopranos