I found this on the internet:

The therapist neutrality concept began with the psychoanalytic idea that if the analyst remains unknown, the analysand uses the analyst as a blank screen for projecting attachment responses to early caregivers. These responses are often the basis for relationship difficulties today, so it can be very helpful to have them revealed for client and therapist to see.

The attempt to offer oneself as a blank screen can be a barrier to successful therapy if it is rigidly practiced without awareness of the client’s individual needs. It can also be an obstacle if the therapist remains emotionally cold and distant as a defense against revealing unwanted thoughts or feelings. Said another way, the blank screen becomes a barrier if it prevents the client from experiencing support from the therapist for healing. My own approach is to find a balance where clients have the opportunity to discover their own thoughts as the therapist does more listening than talking.

Not all schools of therapy use the blank screen. Some, for instance, encourage the therapist to reveal personal responses to model emotional openness. Psychoanalysis may explicitly use neutrality in a disciplined way for clients who are sufficiently secure that they are ready to discover the depths of early childhood emotions and thoughts without distraction. I find that such depths can be revealed in therapy anyway by exploring our interactions here and now, when it’s appropriate in terms of timing, trust, and client capacity.

What I'm hearing here is that the blank slate should be used only if the patient finds it useful.