With ReactiveCocoa bind to inverse of BOOL

There's a signal operator for this, -not.

RAC(self.activityIndicator, hidden) = [RACObserve(self.playButton, selected) not];

map: is what you need.

RAC(self.activityIndicator, hidden) = [RACObserve(self.playButton, selected) map:^id(id value) {
    return @(![value boolValue]);
}];

This transforms the signal into another one based on what you return from the map function.