Hard to say without seeing it. Can't really tell if the top is evenly black now, or if it's the texture of the wood grain or variance in sheen that's the problem. Shellac would act only as a barrier to solve possible compatibility issues and would need to be top coated with either varnish or lacquer. I've finished a few pieces with only shellac and I love working with it, but it's not a durable finish for surfaces that might get hot, damp, or wet. A ring from a tumbler of scotch, for instance, would ruin it.
Sorry if that's unclear. If the top is evenly colored black after a light sanding, I'd go ahead and put on 2 coats of slightly thinned shellac sanding very lightly (when dry) between coats. If you're not experienced brushing shellac, prepare to work fast and resist temptation to rework areas before they are dry as it starts to gel almost instantly. Work wet-into-wet, let it flow off of a good brush, touch up any holidays after it's dry.
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