Alder is not the most cooperative when it comes to dye finishes. It will take the dye unevenly, so you'll wind up with areas where it hardly penetrates at all and others where it soaks in pretty good, giving a blotchy appearance. You'll get better results using a tinted clearcoat if you're set up to do that. Some folks use regular artists' oil paints as a pigment to tint Tru-Oil, since they are compatible by virtue of the latter product consisting largely of linseed oil, which is the vehicle for the artists' paints. But if you're in a position to do a genuine spray finish, you can make a toner using lacquer, poly, or water-based clears colored with universal tints.
I have had good luck using Mixol pigments in lacquer. Their No. 9 Blue is a pretty bright blue, so if you want something more like an ultramarine, you'll want to add a little No. 11 Violet. But you can also use Transtint dye concentrate in lacquer. The Transtint product is also labeled as Colortone by Stewart Macdonald. The blue Transtint/Colortone product is VERY rich blue, nice and deep.
Finally, if the blue aniline dye you already have on hand is soluble in alcohol, you could dissolve the dye at high strength and then make a toner by mixing the dye solution with water-white shellac. You can apply the shellac with a spraygun or Preval sprayer, and then topcoat that with the clear product of your choice. I have successfully topcoated a (untinted) shellac barrier with Tru-Oil and it looks lovely.
