If you don't want to use the Slavic names...
I'd say leave Chernobog as a name for the Black God untranslated. It's close enough to Chernobyl, and the dark (Pripet!) marshes (for those readers who know something of real-world geography) that it probably carries enough negative power.
For the god of justice, use something with the root Fate. It is close enough to the Fates, and to the Fas or the divine law, that the point should carry -- and it also matches the Russian root you are grappling with.
For the White God, try something with the root Lux or light, but stay away from Lucifer.
...These are only suggestions, of course.
