I don’t think it was ambiguous, it just takes a little time to see. That was the reason for the flashback in the previous episode. David Chase just didn’t want to spell it all out for everyone. Bobby specifically said everything would just go black. The only way I think it could be ambiguous is if there were other similar pointed allusions to the end that I may have missed that were intended to introduce ambiguity. Otherwise I think Bobby’s words were right and the series ended at the same instant as the life of its main character. I think more people will realize it as the watch old episodes again and see the bigger picture fitting together.
I think the ending is left to the viewer.