3x3x3 Multi-Blind, also known by MBLD, Multiblind, Multi-BLD, and other similar variants of the name, is an event in which the object is to solve as many 3x3x3 Rubik's Cubes blindfolded as possible after one look at them.
Current WCA Event
In official WCA multi-blind events the competitor first chooses the number of cubes to be attempted. As per regular blindfold events, the competitor then starts by attempting to memorise the number of cubes chosen. The blindfold is then lowered and the competitor attempts to solve them all. The time taken to both memorize and solve all cubes is recorded.
To prevent overly long attempts, there is a time limit of 10 minutes per cube and a maximum of 60 minutes total.
At the end of the event, competitors are ranked by number of cubes successfully solved. If two or more competitors solved the same number of cubes, then they are ranked by total time taken.
If any cubes were not successfully solved, then for each cube not solved, 1 point is deducted from the total number of successfully solved cubes. For example, if an attempt resulted in 4/5 successful solves, then (for the purposes of ranking) the effective number of successful solves is 3 (taking 1 from 4). This would mean the competitor would be ranked in the same group as a solver who managed just 3/3 cubes, using total time taken to distinguish the absolute ranking.