Have you tried putting it down yet? When I was doing a CT that needed a black outline I kept adding and adding and adding and it never got truly black. I was worried because the person I was decorating for was very particular and a real pita!! I finally gave up and said to myself "Well, this is the best I can do".
Wouldn't ya know that once I put that outline down, it was the blackest black ever!

Good luck!!
Edited to add: I used powdered coloring