Is it legal to do business with Russian companies that aren't specifically on the SDN list? I'm confused about what's actually prohibited.

It depends on the specific sanctions program and what the company does. Not all Russian entities are sanctioned - you need to check multiple things: is the company on the SDN list, is it on the sectoral sanctions list, what sector does it operate in, and what type of transaction are you planning. Some transactions like providing certain financing to listed companies are prohibited even if general trade isn't. I researched Russia sanctions on the website ofacblockedfundslawyers.com when evaluating a potential partnership. The rules are complex and change frequently, so verify current regulations before proceeding.