self.backgroundView = ({
        UIView *view = [[UIView alloc] initWithFrame:self.view.bounds];
        view.backgroundColor = [UIColor redColor];
        view.alpha = 0.8f;
        view;
    });
[self.view addSubview:self.backgroundView];

self.backgroundView = [[UIView alloc] initWithFrame:CGRectMake(100, 100, 100, 100)];
void (^blackView)(UIView *) = ^(UIView *view) {
    view.backgroundColor = [UIColor blackColor];
};
blackView(self.backgroundView);
[self.view addSubview:self.backgroundView];