I am doing some black stenciling on a white cake. Any tips on how to get a really good black royal icing?!

Yes I would like to know too!

Royal icing will take color much better than Buttercream because there isn't the fat content that buttercream has (butter/crisco)

But the best advice I can give is to color it brown first, as dark as you can. Then add black (this works great for buttercream too). Takes a lot less color of brown and black.

Also, I highly suggest using Americolor. I use straight americolor black to color my white royal icing and never have any problems getting a true black.