I think the most common approach is ebony dust bonded with superglue. I don't think you can buy "ebony wood filler", unless your luthier just means black wood putty. Typically you hold a scrap of ebony against the belt sander and collect the dust. You dab a bit of superglue into the void, cover it with the dust, and press it in. Repeat as necessary until you've completely filled the void, and then sand it flush and finish.
This method was explained to me by an expert bow guy. I've done it a few times, and for small voids and cracks I find it very effective. Take precautions of course. Both abony dust and cyanoacrylate fumes are nothing you want to breath, and the CA isn't nice to your fingers or to the varnish on your bass. That said, it isn't a difficult tecnique.