You're on the right track but I think you need to concentrate on sanding that filler to an even tone with no dark or bald spots. I have never been a fan of using dark fillers as it just seemed that by the time I sanded the workpiece even in color most of the grain filler was sanded away and felt I just wasted time. I use clear polyurethane as a grain filler when warranted and topcoat with lacquers. I guess I just don't like the exagerrated grain lines and wood pores I haven't done a blackburst like you're going for but the grain in ash pretty much goes black with any stain you apply if you don't first seal the wood. I think a black wiping stain followed by a black tinted topcoat would get you where you want it be. It is up to you to whether you apply in a burst pattern or go for an overall wood grain effect when applying light topcoats of tinted lacquer. The idea of sanding back a dark finish to expose the grain takes a very skilled hand. Inevitably, you wind up with a patchy finish of dark and light spots. Someone just posted a Tele with a finish like you're attempting who inadvertently wound up with a "reverse burst" with a white sand-through band around the perimeter of the body where he "tipped" his sanding block off the edge of the body and sanded out all the color. It's hard to get an even finish by sanding away the color.