How to make less salty Fermented Black Bean Stir Fry

There are two ways to reduce the saltiness of the finished dish: reduce the amount of salt you put in (in this case difficult, as it is in the sauce, so you'd have to put in less sauce) or increase the amount of water that stays in the finished dish. For the second option I personally have the following strategies which may or may not suit your taste:

• add more veggies or tofu or other watery ingredients without salt
• add yoghurt or any other fresh topping that contains no salt
• also make sure that the tofu you add is not too salty, if possible add plain tofu

or as mentioned in the comment to your question you could make your own sauce, one good recipe I found was here: http://www.daringgourmet.com/how-to-make-homemade-black-bean-sauce-or-black-bean-paste/ It tells you how to transform fermented black (soy!) beans into a sauce and also has some interesting examples how you can use it and it doesn't seem very complicated to do.

Of course if you are adventurous you could also try fermenting soy beans yourself, but that's another story.

Depending on the brand of black bean sauce, the recipe given misses a few balancing elements, which might also force you to add in more than what is called for of the black bean sauce (thus oversalting the dish).

A stir fry sauce tends to need a sour element (light or black vinegar, tomato paste, lime, fruit juice) and some sweet element (sugar, syrup, honey, fruit) even if it isn't a literal "sweet and sour" sauce (which adds larger amount of both these elements and balances them). If you rely on the black bean sauce (which is there mainly for umami, salt and aroma, but can contain some sugar and vinegar) to fill that role, the issue described likely happens.

Starches are bland or neutral in flavor and can soak up quite a bit of salt without tasting over-seasoned - that's one reason adding potato is a common suggestion for over-salting a dish.

In this case, since the sauce is based on black beans and garlic, I would suggest unseasoned black beans, pureed, should take up a bit of the salt while keeping fairly close to the original flavor profile. Or, looking at the recipe Michael Paul found, perhaps soybeans (optionally black or fermented, if available) might be a better match, flavor-wise, if you have any to hand.

You're essentially diluting the sauce with the bean paste, so the other flavors may also become less distinct - so you can, if you wish, include more garlic (or any other overt flavors in the sauce) for balance if the sauce's flavor becomes too diluted along the way.