Hi, thanks you both for your answers,
My tries with al_draw_tinted_bitmap() failed because I was putting 0 for r, g and b, I was only taking into consideration the alpha ; thanks!
But unfortunately, it doesn't change my problem with the "dark zone" thing (now draw directly on the buffer + al_clear_to_color()) ; I try, with and without it, and with al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_INVERSE_ALPHA) (default) and al_set_blender(ALLEGRO_ADD , ALLEGRO_ONE , ALLEGRO_ZERO)
In both case, the background map disappears, I don't understand why, the blender don't blend the black screen and the transparent light image? apparently no since the transparent become white.
I was using the al_set_blender() in my game loop (in the draw section), but I saw you using it in your display setup at the beginning ; is it important? (I try moving it, but apparently, nothing change)
Thanks in advance ; sorry again for these questions..