A trial gives one group a drug and the other a placebo, and neither the participants nor the staff measuring the outcome know who got which. Why is keeping the STAFF blind necessary, given that the participants are already blind?

B. Because an assessor who knows the allocation can shade judgements.

Single blinding handles the participant's expectations and leaves the assessor's untouched, and ratings of pain or mood are full of borderline calls a believer resolves in one direction, without cheating, because that is how expectation acts on judgement. There is a second route too: an assessor may probe one group more encouragingly, so the procedure stops being identical across arms.