Preparation - If a given recipe doesn't specify how to prepare douchi before using them, I strongly suggest that you soak them for about 5 minutes in cold water, then rinsed and briefly soaked again in 2-3 changes of cold water.  If you're unsure if you've soaked the beans long enough, try tasting one – it should be strong and salty, but not overwhelmingly so.

One important note to get out of the way: douchi are NOT the same as the black beans that you'll encounter in, say, a burrito.  The black bean (or black turtle bean) of Central/South American cooking is a variety of the common bean, Phaseolus vulgaris.  It's sold canned or dried, and is neither fermented nor salted.  Douchi - Chinese black beans - are cooked, fermented, and heavily salted black soy beans (Glycine max).  They are both beans, and they are both black, but the similarities end there.  The two have wildly different flavours and uses, and cannot be considered even remotely interchangeable.

Chinese black beans can be somewhat tricky to find on their own, so it's worth exploring substitution possibilities.  That being said, we can't venture too far afield in our search, as no other ingredient has a similar flavour.  Chinese black bean sauce (豆豉醬 - dòuchǐjiàng) can substitute for the beans themselves, but this is a somewhat like substituting tomato sauce for tomatoes.  The sauce has added ingredients (usually garlic, oil, chilies, and soy sauce), but has the advantage of being basically ready-to-use right out of the jar.  That being said, you have more control over your final dish with the douchi beans themselves, so it's worth using them if you have the option.

It should be noted that we're talking about substituting specifically with Chinese salty black bean sauce; there is a different sweet black bean sauce called tianmian sauce (甜面酱 or 甜酱 tiánmiànjiàng/tiánjiàng) that's considerably sweeter, and made with additional ingredients.  This sauce has also been adopted into Korean cooking, where it's known as chunjang (춘장).  Tianmian sauce is considerably sweeter than standard black bean sauce, and will contribute a very different overall flavour (closer to hoisin sauce) to any recipe in which it's used.  While it is delicious, tianmian sauce isn't a good substitute for douchi.

I have read that Nigerian iru is similar to douchi in many respects.  The product, which is made from fermented locust beans, certainly has a degree of visual and biological similarity, but I can't personally comment on the overall similarity in flavour myself.  If you've tried both ingredients, I'd love to hear from you!

Soybeans are one of the worlds most economically important crops, and have been subject to genetic engineering for many years.  In 2014 it was estimated that approximately 82% of the world's total soy bean production (by area) focused on GM soybean products.  Most genetic engineering has focused on making the crops resistant to the herbicide glyphosate, though there has also been some work done to improve oil yields.

Given the prominence of GM soy, you might be surprised to learn that douchi are almost certainly NOT made with any GMO beans.  The vast majority of soybean crops are grown for the production of soy oil.  Black soybeans do not produce a great deal of oil, and are primarily consumed whole (or converted into products like douchi).  Black soybean varieties have, to date, only been subject to conventional breeding practices.

Disclaimer - the information here is meant to act as a guide, and should not replace the advice of a trained medical professional.

Fermented soy products like douchi generally contain high levels of the chemical compound histamine.  This chemical is part of a group of important organic molecules called the biogenic amines, and they play a complex role in our food and health.

Biogenic amines are found in a huge array of foods, and are particularly prominent in fermented foods.  These compounds play many important roles (both in the body and in our food), but can have toxicological effects in high quantities.  Histamine, for example, helps regulate acid production in the stomach, but can also cause inflammatory and allergic responses.  About 1% of the population deals with histamine intolerance, which is characterized by a wide range of inflammatory and digestive distresses.  People suffering from histamine intolerance are generally advised to avoid soy products, as well as other foods like beer, wine, hard cheese, citrus, smoked meats, and more.