Černobog (black god) and Belobog (white god) are recent „inventions" of some neopagan rodnovery fractions. Slavic pantheon knows no ewil god or god of ewil. Chernobog has some atributes of Veles and Belobog some of vise Stribog. Also, it is good to know that slavic pantheon have several...
The attitude is everything :) I am Serbian and speak six slavic languages but cant connect your „arguments" with the quotation. Facts are (by all researchers and old historians) that slavic beliefs are based on their descending from old gods.
Not exactly true story and position in pantheon (slavic pantheon „organisation"is quite different from greek) but good text all the same.
http://www.slavorum.org/perun-slavic-god-of-thunder/