Blending is not done with FBO textures.

Hi! I draw a red rectangle on the first FBO and a green and a blue rectangle on a second FBO, but, the alpha blending is not done, it doesn’t draw the red rectangle and at the intersection of the blue and the green rectangle the alpha blending is not done.
It works when I don’t use FBO.
Here is a simple code using the SFML library which reproduce the problem.
Driver bug ?
I don’t have this problem on my laptop with mesa driver (on linux) and an amd graphic card.

int main(int argc, char* argv[])
{
    sf::RenderWindow window(sf::VideoMode(800, 600), "test blending", sf::Style::Default,
    sf::ContextSettings(0, 0, 0, 3, 0));
    sf::RenderTexture rt1, rt2;
    rt1.create(800, 600, sf::ContextSettings(0, 0, 0, 3, 0));
    rt2.create(800, 600, sf::ContextSettings(0, 0, 0, 3, 0));
    sf::RectangleShape tile1 (sf::Vector2f(100, 50));
    tile1.setFillColor(sf::Color(255, 0, 0, 50));
    sf::RectangleShape tile2 (sf::Vector2f(100, 50));
    tile2.setPosition(50, 25);
    tile2.setFillColor(sf::Color(0, 255, 0, 50));
    sf::RectangleShape tile3 (sf::Vector2f(100, 50));
    tile3.setPosition(-50, 25);
    tile3.setFillColor(sf::Color(0, 0, 255, 50));
    sf::Sprite trt1(rt1.getTexture());
    sf::Sprite trt2(rt2.getTexture());
    while (window.isOpen()) {
         sf::Event event;
         while (window.pollEvent(event)) {
             if (event.type == sf::Event::Closed) {
                 window.close();
             }
         }
         window.clear();
         rt1.clear();
         rt2.clear();
         rt1.draw(tile1);
         rt1.display();
         rt2.draw(tile2);
         rt2.draw(tile3);
         rt2.display();
         window.draw(trt1);
         window.draw(trt2);
         window.display();
    }
    return 0;
}

Someone told me that there are other parameters for blending with FBO texture but I don’t know which parameters to set.