When a character is bei g made to lick dirty feet or shoes, it should be graphic, detailed and disgusting. It is humiliating even for those with a fetishism and should make them feel disgusted at both themselves and what they have to do. Remember its a sex act , just kink one. Go into detail with lots of background on why the shoes are dirty; if they've been walking around urban areas with concrete a it will be general dirt making their tongues go black, as well as oil, dust, things discarded by people, (a minor part) bodily fluids, refuse, dirt, the asphalt etc. If its in an area like a park, forest or garden, it's dirt and more mud based which can be thick and sticky or more dusty depending on the story. There might be shit in both too. Describe the feeling. It will taste disgusting and vile, it will be gritty between their teeth and as they swallow make them nauseous and want to throw up. Use lots of varied and descriptive words, like grime, filth, mud, dirt, dust etc