According to Christoph Waltz he is not playing Blofeld, but another villain entirely. Here is the article: http://www.slashfilm.com/christoph-waltz-blofeld-denial/
I only hope they are able to get a good actor who is just right for the Blofeld role, and that actor is willing and able to come back for a few more films like the actors playing the M.I. 6 office staff do. I would not be surprised if Blofeld only shows up as a rarely scene background character (like in From Russia With Love, and Thunderball), and does not receive any real screen time or become the main villain until Bond 26. They might not even tell anyone who the actor is until the release on Bond 26.