The blank-slate concept I believe is my T's way of helping the transference happen by allowing me to see her in whatever way I needed to, like a mother, sister, or friend. Her personality, emotions, and reactions to things are genuine but she tells me very little about herself and she doesn't often share any of her experiences that may relate to mine. When I asked her about this (finally) she explained that this time was supposed to be about me and not her but I was welcome to ask her questions. So at least then I didn't feel like she didn't trust me or something.
When she did share a few more things about herself it really helped me relax and trust her more (and I told her this.) It was too easy to idealize her and think her life was way better than mine so she couldn't possibly really understand what I was going through. And one time when she shared something about herself that she has in common with me I swear I heard an almost audible "click" of connection and mutual understanding.
When I asked her what transference was was when she brought up the blank-slate idea. I can see why this is part of her standard operating procedure but I am very susceptible to transference and it would have happened no matter what. So for me the blank-slate has been frustrating but for others, it may be what they need to allow the transference to happen. I guess we don't know until we get there.