Several people have asked me about block randomization, and while I discuss it briefly on one of my web pages,
there is a better definition on the Consort Statement web site that defines several variations of randomization including block randomization.
Blocking can be used to ensure close balance of the numbers in each group at any time during the trial. After a block of every 10 participants, for example, five would be allocated to each arm of the trial (105). Improved balance comes at the cost of reducing the unpredictability of the sequence. Although the order of interventions varies randomly within each block, a person running the trial could deduce some of the next treatment allocations if they discovered the block size (106). Blinding the interventions, larger block sizes, and randomly varying the block size can ameliorate this problem. www.consort-statement.org/examples8b.htm
It is worth noting that a process that some people use, creating a separate randomization list for different centers or for different demographic groups in order to assure good balance across these important factor, is actually stratified randomization and not block randomization.