Soot is a black powdery substance that covers surfaces after incomplete burning of fuel. When something has been covered in soot, it appears completely black or dark in color. Therefore, some synonyms for the phrase "most soot covered" could include "blackened," "sooty," "gritty," "grimy," "smudgy," "sullied," and "stained." Other expressions that could be used to describe the same concept include "coal black," "charred," "grubby," and "smoky." Depending on the context and the object being described, there may be further synonyms that could better communicate the extent to which it is covered in soot.