This is the literal dirt, soot, or grime that soils a surface. Think of a muddy smudge on your clean white trousers or a blemish made by dirt on a pristine wall.
Using "smirch" instead of just "dirty" or "bad" adds a layer of weight and history to your writing. It implies a loss of purity or honor, making it a powerful tool for writing concise prose or creating compelling story blurbs . Degrees of Concision: A Note on Writing in General smirch
(verb): To make something dirty, stained, or discoloured. Smirch (noun): A stain, blot, or blemish on someone's reputation or character. This is the literal dirt, soot, or grime