the Small Shield in one's profile and in the list of clan members comes in 3 states,

 - **Red** - the player has Opt-Out of Clan Wars regardless of Trophy Count
 - **Green** - the player as Opt-In of Clan Wars
 - **Black** - the player is ineligible for Clan Wars

while i have never seen the Black Can in my own profile, when i have seen it with other members it has normally been for new members who had came from a clan. there is also a small bar under the Black Shield which is how long that person still has to wait before they can choose to Opt-In/Out of clan wars.

Should Note that a Clan Leader can override a player's Clan War Prference

> **Note:** Leaders can still choose players that have chosen to opt-out by default, and can always leave out players that have chosen to opt-in by default. 

Source: [Clan Wars - Eligiblity of Members Point 2][1]


  [1]: http://clashofclans.wikia.com/wiki/Clan_Wars