this is a q and a from the aums that might help or you could use a panel (black)
and decrease or increase its alpha


Q: How can I smoothly fade one view into another?
A: Use the code below:

view second_camera{}
action set_second_camera // place any entity in the level and set its proper position and orientation
{
my.invisible = on;
my.passable = on;
wait (3);
second_camera.pos_x = 0;
second_camera.pos_y = 0;
second_camera.size_x = screen_size.x;
second_camera.size_y = screen_size.y;
second_camera.alpha = 0;
second_camera.transparent = on;
camera.alpha = 100;
camera.transparent = on;
second_camera.x = my.x;
second_camera.y = my.y;
second_camera.z = my.z;
second_camera.pan = my.pan;
second_camera.tilt = my.tilt;
second_camera.roll = my.roll;
}

function toggle_cameras()
{
second_camera.visible = on;
while (second_camera.alpha < 99)
{
second_camera.alpha += 2 * time;
camera.alpha -= 2 * time;
wait (1);
}
camera.visible = off;
second_camera.transparent = off;
}

on_t = toggle_cameras;