BLending and Transparency

Hi all,

I'm experiencing a strange issue.
I display an image which is a textured quad in RGB format.

I try to display a transparent rectangle over it.
I use the following code:

glPushMatrix();
  glEnable(GL_BLEND);
   glBlendFunc( GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA );
    glColor4d(0.0, 0.0, 1.0, 0.5);
    glRecti(rc.left(), rc.top(), rc.right(), rc.bottom());
  glDisable(GL_BLEND);
glPopMatrix();

It works fine.
But with some images, the rectangle is not transparent blue, but BLACK
transparent...
And I just can't figure out what's the trick.
It is a simple PNG image (RGB) and it is just well displayed on scrren
though.

Any idea or suggestion ?

Thanks.

Zyend