Help! my horse has a weird skin rash thing and I need to know how to treat it!?

Here's the deal. The horse that I ride and love (but do not own) has something weird going on with his legs. He gets these black dot things that look like little dirt but they stick to his fur and you can't just brush them out I usually have to pick them out with my fingers. He's a fleabitten grey TB in case that helps. I Gave him a full on bath yesterday and found that they still didn't really come out. The water pressure brought a few of them out but there's still tons on his legs that i can't get rid of. This only happens on the lower half of his legs. so it's not like going up to his shoulders or anything. I know he doesn't have rain rot cuz his coat isn't all gunky and matted and his hair doesn't fall out when i groom him. I tried coating him with Furizone after his bath and i also caked corona on his back legs afterwards. I haven't gone out to the barn yet to check on him. I didn't try MTG but i might. please tell me if you know what this is or how to treat it. thanks!