How to make authentic black Russian bread? [duplicate]

I'm trying to re-create the authentic black Russian bread that was first mass produced during soviet times. Nowadays you can find it in any Russian market and many international supermarkets in the US.

The problem is most recipes I can find online either:

1. Use a lot of additives such as cacao powder and coffee to color the bread, which don't seem historically plausible

or

2. Don't achieve nearly the same dark shade as the pre-packaged store variants

Anecdotally, my dad claims that this bread was sold in the Soviet Union in his youth as the cheapest bread available, for those who couldn't afford bread made from finely milled wheat. At its initial inception it was mass produced by the state and supposedly didn't taste great (although it did have that dark black color). Considering it was the cheapest bread around, it seems highly unlikely it was created with cacao or espresso powder in soviet times.

Eventually, people started prizing this bread more and recipes evolved until we got the modern variant which most people (at least in Russia) find appetizing. It's possible the modern variant uses the more expensive color impacting ingredients, but it seems more likely that some trick that was used to make the original "dirt cheap" black bread is still employed or borrowed from today.

I’ve made a number of different variants following the recipes online and overall I’m happy with the results in terms of taste, but it’s driving me crazy that I can’t figure out how to make the loaf authentically black.