I've read a thread here about using, I think, blue coloring to turn chocolate frosting black. Do I remember that right?

I believe you use black color to chocolate buttercream to make it black.

Mac I posted my answer in the other post you made with this question. Like I said in the other post, I think blue would work perfectly fine if the chocolate frosting was a nice dark chocolate. But if it's a light colored chocolate frosting (like milk chocolate) I would use the black.

I tried adding black to light chocolate frosting and it didn't work out well for me to get that nice rich brown color. I instead found a darker cocoa to use to get a richer brown chocolate color and flavor.