I've been given a collection of 'Milliput' 2-part resin fillers in various colours - I used the white version to repair a mobile leisure washroom basin for a Customer - there's a black version available:
https://www.milliput.com/black.html
I hope this helps.
Best wishes
Guy
(Usual disclaimer i.e. I have no commercial connection with this company, I'm just a satisfied user)