Osemsmerovka č. 12 406 - I am black when clean and white when dirty. What am I?

T R O P E R A T A S S T O C V H M R E T U B N R I T U A L E B G C E R R E E R R S D A H D E E H C R T M M R S I R M A A I E E A U O S U A V T A L X P C S E N E U L G D O O K Y R N T B G E T D P B R P E S T H C A Y E O U A R R P O W E R D R P