I think you are supposed to start with a chocolate BC frosting. Then you add the black coloring. Let it sit overnight so that you can see if you need to add more black. It may lighten up as it sits.
I don't ever use a base color for coloring the royal icing black...I just use a lot of black....add a little, mix, add a little more, mix, add a little more mix. It's true that the color will come in more after some time but I never wait for that time to come...I just add the black until I get the desired tone.
hth
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