To render black screen with alpha
use ui::Layout Like View.
setBackGroundColor and setOpacity
Let's see the sample code.

ui::Layout* blackView = Layout::create();
blackView->setContentSize(Size(DISPLAY_WIDTH,DISPLAY_HEIGHT));
blackView->setBackGroundColor(Color3B::BLACK);
// need backGroundColorType
blackView->setBackGroundColorType(Layout::BackGroundColorType::SOLID);
blackView->setOpacity(150);
blackView->setVisible(false);
this->addChild(blackView);
		

Point is setBackGroundColorType
code is

void Layout::setBackGroundColorType(BackGroundColorType type)
{
    if (_colorType == type)
    {
        return;
    }
    switch (_colorType)
    {
        case BackGroundColorType::NONE:
            if (_colorRender)
            {
                removeProtectedChild(_colorRender);
                _colorRender = nullptr;
            }
            if (_gradientRender)
            {
                removeProtectedChild(_gradientRender);
                _gradientRender = nullptr;
            }
            break;
        case BackGroundColorType::SOLID:
            if (_colorRender)
            {
                removeProtectedChild(_colorRender);
                _colorRender = nullptr;
            }
            break;
        case BackGroundColorType::GRADIENT:
            if (_gradientRender)
            {
                removeProtectedChild(_gradientRender);
                _gradientRender = nullptr;
            }
            break;
        default:
            break;
    }
    _colorType = type;
    switch (_colorType)
    {
        case BackGroundColorType::NONE:
            break;
        case BackGroundColorType::SOLID:
            _colorRender = LayerColor::create();
            _colorRender->setContentSize(_contentSize);
            _colorRender->setOpacity(_cOpacity);
            _colorRender->setColor(_cColor);
            addProtectedChild(_colorRender, BCAKGROUNDCOLORRENDERER_Z, -1);
            break;
        case BackGroundColorType::GRADIENT:
            _gradientRender = LayerGradient::create();
            _gradientRender->setContentSize(_contentSize);
            _gradientRender->setOpacity(_cOpacity);
            _gradientRender->setStartColor(_gStartColor);
            _gradientRender->setEndColor(_gEndColor);
            _gradientRender->setVector(_alongVector);
            addProtectedChild(_gradientRender, BCAKGROUNDCOLORRENDERER_Z, -1);
            break;
        default:
            break;
    }
}

if set BackGroundColorType::SOLID
create LayoutColor object and render color.

This is available in cocos2dx 3.15.1