At least you know it will be as black as it can be. Rit dye comes in many different colors, I have only used black so far. If the process works well with black, the other colors should be a piece of cake. I have been told that a true black color is the hardest to obtain. If your part has a slight purple tint that is not dark black, it could have been caused by the part not being anodized completely, too short of a dying time, or the alloy type of the aluminum.
Cast aluminum is hard to anodize and dye.
Rit dye is not light fast and may fade in direct sunlight. I don't plan on leaving my scopes outside in direct sunlight everyday of the year. An anodized mail box dyed black with Rit dye might have a problem. There are professional black organic dyes that are light fast. I am now using one of these dyes.