+1 on the epoxy and black powder approach. I use graphite powder but that is likely about the same as the charcoal ... which is a great idea I'm going to have to try. Also I like the idea of using shellac to protect the surrounding wood though I haven't done that. In a prior post a few years ago Peter Quinn suggested (hope I'm paraphrasing this correctly) that just putting clear epoxy into a imperfection worked fine since the "depth" trapped light and gave the appearance of being dark. My only caution would be that some waterborne finishes don't adhere well to epoxy. So test your finish.