Some experiments refuse to end. Before I deemed it a failure, I bottled up most of the the black walnut nocino into a fancy bottle. But there was some of the vile green liquid left over, and I just left it in the large jar in which the green walnuts have macerated. This meant it was exposed to a lot more oxygen than previously, when I had filled the sealed jar basically up to the top. Turns out oxidation was exactly what the nocino needed to turn a beautiful espresso color:
It still tasted a bit medicinal, but some carmelly undertones had developed that made the liqueur somewhat smoother. I’m going to aerate the rest of the batch and see how it starts to evolve. At the very least, the color will no longer resemble bilge-water. Quite an improvement!