I've also seen thrush that has a black, tarry appearance and can inundate the creases alongside the frog, any loose flaps of dry skin at the back of the heels, or anywhere the hoof is packed w/manure turns thrushy, fast. It does stink, bad, too!
Clean the hoof really well and then paint on Koppertox, Thrushbuster, Thrush-X, even bleach like MA said, those will help.
Keeping the hooves clean and moderately dry will help immensely. I have read that letting them run in clean mud is not a problem, it is actually part of how a horse's hooves are designed to work. It is when they stand around in dirty mud (i.e., urine, feces and decomposing feed) that it begins to cause a problem. My farrier agrees w/the clean mud. He digs the mud out of my horse's hooves when he comes to trim and while I can honestly say I rarely clean their hooves, he most times say they look really good b/c I try to keep them on clean footing.
I DO clean hooves frequently on my driving horses, though, and often enough to check for problems, just not daily, sometimes not even weekly.
Liz M.