Allocation concealment and blinding are often confused because both involve hiding information, but they operate at different moments and protect against different problems.
Allocation concealment protects the assignment sequence before and at the moment of randomization. It means the person enrolling a patient cannot know or reliably predict which arm that patient will receive. Practical forms include a central randomization service, sequentially numbered opaque sealed envelopes, or a locked computer system that reveals the assignment only after the patient is enrolled. Concealment matters because enrollment is a decision: if the enrolling clinician can predict the next assignment, they can — consciously or not — delay a sick patient until the next control slot, or enroll a healthier patient when the next slot is the treatment arm. That behavior is selection bias, and it corrupts the very comparability randomization was meant to create.
Blinding begins after assignment. It means participants, providers, or outcome assessors do not know which treatment a given patient received. Blinding protects against performance bias (differential care or behavior once treatment is known) and detection bias (differential measurement or reporting of outcomes).
So the two safeguards answer different questions. Concealment asks: could the assignment have been influenced at enrollment? Blinding asks: could knowledge of the assignment have influenced care or measurement afterward? A trial can conceal allocation perfectly and still be unblinded, or claim blinding while using a predictable allocation list — the two are independent.