Ebonite 'powder' plus superglue is great for filling in substantial holes, chips, and wide cracks, but i doubt that it is that suitable for a scratch. I think the grains of ebonite powder may be too coarse, and not go much into the scratch. This is partly why I suggested superglue on its own. You cannot mix superglue and powder before using it, and I think you could get into a heck of a mess using them alternately on a long scratch; superglue readily dissolves the surface of typical ABS clarinet plastic.
Unless superglue is dabbed very carefully and splaringly along the centre of a scartch with say a dental probe or equivalent, I think you could easily finish up with more cosmetic damage than what you started with. BTW, that is how I apply thin superglue to narrow splits in timber clarinets - no filler.
On the other hand, the very fine, black, paint pigment powder from a paint or hardware store, can be mixed with epoxy, and may more readily fill the scratch..