I'm trying to make black royal icing and have used a big tube of the black Wilton gels and it's still a dark gray/purplish colour. Will it darken if I let it sit, like BC? Or should I keep adding more black?

I don't think royal darkens. It will help to add a little brown.

All icing darkens over time. The only way I have ever achieved true black is to make it several days ahead and it will deepen - the same applies for red. I like to make the icing in advance whenever possible since the colors change.

I am, however, the LEAST organized person on the planet so it hardly every happens.