As the name implies, the Rovner Dark ligature is dark. In fact, it's been my experience that Rovner ligatures have an effect of dampening my sound.
A ligature can go only so far in darkening your sound.
A better option may be to consider trying other reed cuts. What kind of reed are you using now? A thick-cut reed (ie, more "wood" in the heart) -- often thought of as a classical type of reed -- will help to give you a somewhat darker, fatter, sound and a stronger tonal core. I'd suggest Alexander Classique and Gonzales reeds. They are made from very high quality cane. Vandoren Traditional reeds are another option but they are far less consistant than Alexander or Gonzales. Some classical players like Hemkie. They can be okay but it's been my experience that they don't last very long.
Perhaps the best suggestion that comes to mind is to check is to see if you're taking enough of the mouthpiece beak into your mouth. Turn the mouthpiece sideways and see the spot where the reeds separates from the facing curve of the mouthpiece. That's where you want to place your lower lip. If you're taking in less of the mouthpiece you are, in effect, choking your sound and it can become brighter. It's like using your strength of reed on a smaller tip opening. I'm amazed at the number of saxophone teachers who don't teach this.
Hope these suggestions can be of help.
I also agree with the other comments made about having a tonal conception. That, along with practical work on your horn, goes a long way to getting your sound to where you want it to be.