I tried adding black coloring to make the a dark black, but that didn't work - it hardened up and became crumbly..

Any suggestions or is the only alternative royal icing?

1.  To color chocolate you need either an oil-based color made for chocolate or powdered color.

2.  I haven't used W black chocolate, but I have used Klausen black and CK black and both are crazy black.

Wilton black candy melts are as black as it gets. I use them all the time. I'm not sure what the issue you're having is, but they're definitely black.

I agree they are black.  So, I made a small batch of black royal icing and it is a lot darker black then the melts.  Was trying to cut corner here, but obviously not going to work for me.  I am doing stencil work on a cake and want that really deep black.

Thanks for your responses....