That’s just going to be an epoxy resin. Your typical 5min epoxy from the hardware store is a bit on the thick side for actual carbon layup, but it makes gap filling easy. Less is more, or you’ll end up with a goopy mess with lumps all over. Might require a few thin layers to build it up to where you can sand everything smooth.
You can dye the epoxy black, if you so desire, using Rit powdered dye.
If you’ve never used epoxy before, the key is to make sure your mixture ratio is accurate and that you mix very well. If you don’t mix well, it won’t set or cure properly and will be sticky forever.