A.34.1- The ability is written "Shadow". [CompRules 2001/07/23 - 502.8]
A.34.2- A creature with Shadow cannot be blocked by creatures without Shadow, and a creature without Shadow cannot be blocked by a creature with Shadow. [CompRules 2001/07/23 - 502.8b]
A.34.3- Shadow is a non-cumulative ability. Having Shadow more than once gives no additional effect. See Rule A.1.2. [CompRules 2001/07/23 - 502.8c]
A.34.Ruling.1- Shadow includes an evasion ability (see Rule A.15), so it is cumulative with other evasion abilities. [CompRules 2001/07/23 - 502.8a]
A.34.Ruling.2- It is possible for Banding to result in a Shadow creature blocking or being blocked by a non-Shadow creature. This is not illegal. [D'Angelo 1998/02/03]