No, the Courier is not a blank slate at all and the writing in FONV forces you into a very specific role. In addition, the story isn't even interesting except for people who like revenge in a spaghetti Western environment. I like some spaghetti Westerns and grew up with some of them, but I am not interested at all in roleplaying in such an environment when the game claims to be post-apocalyptic (and is not). Even for what you stated, it makes no sense at all as FONV is written for the Courier to be asking NPCs about the NCR, Legion, etc because s/he would have more knowledge about such matters as well as other stuff in the world context than most NPCs (i.e., Couriers are better traveled and more worldly than typical people due to the nature of the job itself).
The most blank slate that FO has ever gotten would probably be FO3 because you can create any type of background personality you want regardless of the intro and tutorial. You could be a person who hates your father or even holds an irrational grudge against your mother for dying while bearing you into the world (such a wonderful world it is, after all). You cannot do such personality/character construction with the Courier in FONV and have it make any sense, nor does it make much sense for the first two games.
Also, roleplaying is defined as "playing a role". No, not every game that has you play some character or characters is roleplaying because many games do not actually give you any details of the role you would be playing (i.e., they are not roleplaying games). Being "Doom Guy" in Doom is not roleplaying. Being Gordon Freeman is not roleplaying since you really do not have much details about him, nor is there any element of his character progression during the game (aside from getting different weapons which hardly counts as character progression). Same with action games like Blood Rayne or fighting games like Street Fighter, Mortal Kombat, Dead or Alive, etc. Same with racing games, puzzle games, etc. Roleplaying games require that you be given a role and follow progression of that role through the game. They may have a fixed character or characters where progress follows one or more story arcs or it may be that you create some type of role from the limited assets offered and progress follows what you do with the character (i.e., BGS games).
All of this is moot for FO4 as BGS has decided to (finally) try adopting modern standards for roleplaying by offering some type of structured background in order to attempt answering criticisms about their writing (as well as trying something new for them, a standard policy for BGS and Todd's team). Whether or not they succeeded in doing this with FO4 won't be known by anyone until the game is out and played for awhile, and even then it will be a largely subjective appraisal.