Blocking - as it pertains to Memory, is when existing memories interfere with the encoding or retrieval of a desired Memory (Wikipedia/Sins_of_Memory). Blocking is one of the seven Sins of Memory proposed by Daniel Schacter. See also Interference Theory that further elaborates on types of Blocking referred to, in that theory, as Interference.

In Learning Theory, Blocking pertains to the inability for a second Conditioned Stimulus (CS2) to elicit a Conditioned Response alone. For it to be considered Blocking, CS2 must be introduced as a pair in a subsequent phase of Conditioning Trials (CS1 + CS2 now predicts the Unconditioned Stimulus). Had the CS2 been introduced in the first phase with CS1 or alone, it could have elicited a Conditioned Response. When introduced in the second phase, however, it is blocked from gaining operant control.
Barnes #RA15