I am an amateur violin builder with little to no experience in the finishing department. My latest customer requested a transparent black finish on his instrument.
Traditionally, violins are finished with oil varnish. 2-3 clear coats (to prevent botching), followed by 6-7 color coats, followed by 3 more clear coats. I rub out Each coat with ground pumice and rotten stone as prescribed by my violin building book by Henry strobel.
For this fiddle I really want to pop the figure. The best results I've found so far for achieving this is to dye the whole piece and sand it back 2-3 times until I get the contrast I'm looking for. The dye will stay in the curls/figure but will be sanded off of the straight grain completely. On a flat piece of wood, I can definitely see how this would work. On an arched surface however, there is more than just the end grain of the curl that will soak up the dye. The whole piece is full of end grain. Furthermore, the body of a fiddle is to be graduated to a very specific thickness in order to produce a decent tone. Too much sanding to get back to bare wood will affect these graduations.
My next method to try is to pop the grain with boiled linseed oil then seal (can I use shellac over blo?). From there, I would have to apply 7-10 coats of black oil varnish followed by 3 clear coats. I think this method will work but I imagine it to be a little too transparent for my liking. I may be wrong.
Whatever I do on the back(curly maple) has to be matched on the front(spruce). There's no figure about the spruce but it is arched the same as the back. So, more end grain to contend with.
Without a doubt I need to seal the wood before adding color or else I'm going to have a blotchy mess. I need the figure to pop. And I need a good deep black color through which you can see this popping figure. Since I don't think(and again, I may be wrong) that the black oil varnish will be rich enough, Is there a coloring agent that can be applied after the sealer coat and before the oil varnish?
Thanks in advance for your help
Any other method suggestions are also welcomed

I'm pretty sure it will take more than a reply on this forum to get all those questions covered. It's rare to see any input from an experienced luthier here. I suggest you start with luthier forums, and what ever you try to use, work on scrap for the first 100 experiments until the technique is perfected.I use that black dye and sanding back on my artistic woodturning. I hardly produce any wood dust, I can't imagine I thin the wood more than a few thousandths. I might add that I hate boiled linseed oil. Hobbyists brag about the use of it, but there is a reason that it fell from favor decades ago. It might look good the week you use it, but it oxidizes and gets really dark years latter.

Consider a dewaxed shellac as a sealer. In my experience it enhances and highlights. Do a test sample, you may find the blo not necessary.

Here's a close to finished product.

The method
1pt instant coffee to 1pt hot water. Flood surface, wipe dry.
Combine steel wool and vinegar and let sit for a week or so. The steel will disolve.
Brush mixture over the previous coffee coat and watch your project blacken within seconds.
I then applied my first coat of shellac.
Next I sanded the shellac and in spots, sanded through to bare wood for a weathered appearance.
Apply 2-3 more coats of shellac. Sand in between each coat.
That's where I'm at now. The next step will be an amber oil varnish on the back and sides so that they match the front followed by 7 coats of clear oil varnish. I use pumice and rotten stone to rub out in between coats. Hope this helps someone

View higher quality, full size image (3264 X 1836)

View higher quality, full size image (3264 X 1836)

View higher quality, full size image (3264 X 1836)

I would just French Polish it. It will make the grain really 'pop'. I think back-in-the-day all that they had was shellac - a great finish for a violin.

"Violin varnish is applied with a brush in two or more coats. It is composed of sun-thickened oils such as linseed or walnut, oxidized resins and added coloring matter. Good historic Italian varnish is highly translucent and soft, yet friable." Personally, I feel that coffee and iron oxide finish to too opaque for a fine instrument.

Varnish Secrets