Black soy sauce is the Thai version of dark soy sauce. It's not very salty and is used mainly to add a delicious-looking brown colour to food. It has a strong molasses-y flavour, so it's usually used sparingly. Chinese dark soy sauce is a great substitute, but it is saltier than the Thai version; so if using more than just a dash or two, you may want to reduce other salty ingredients slightly. Tasting and adjusting is key.