You need to use a variable with a .
(dot). See Understanding Scopes in AngularJs
<ion-slide-box show-pager="false">
<ion-slide>
TEST111111111111
</ion-slide>
<ion-slide>
TEST222222222222
</ion-slide>
<ion-slide>
TEST333333333333
<button class="button button-dark" ng-click="someObject.test = !someObject.test">
Test
</button>
Works > True or False? > {{someObject.test}}
</ion-slide>
</ion-slide-box>
Does not work > True or False? > {{someObject.test}}
And make sure that in your controller you also initialize a value for someObject
$scope.someObject = {
test: false
};