Well, actually you can make it darker without editing the textures. Just change the RGB values under "color" inside .mtl file. try with ["color" : "0.2,0.2,0.2,1.0", ] it should make it a bit darker, just play around with these values. Make sure all RGB values are the same if you don't want to change it's color.
You may already know that the last one is the transparency.