How to make authentic black Russian bread? 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:

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

or

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 &quot;dirt cheap&quot; 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.
There are two elements responsible for the darkness of “black bread”, which is a term used for different breads all over East, North and Central Europe

Whole grain flour, typically rye, but sometimes with a part wheat or spelt.
A low and slow bake, the probably most extreme example is Pumpernickel, which needs almost a day at 100-120 C and is more steamed than baked. Other recipes aim for 2.5 hours at 150 C. Above that, you will still get a good and dark bread, but more brown-grey than dark “black”.

The lower and slower, the darker will the bread turn out. Some sources claim that the darkness is caused by the Maillard reaction, but I am not sure whether this is the whole truth. Considering the temperature range, other factors like enzymes out of the grain are at least involved. The only “traditional” colorant I could find at a quick research was molasses or similar, which is used in some recipes.