If you've tried all of that, then it's not a mark left by the rubber of the float. Does your grout float have a metal backing plate on it, by chance? Your "black marks" could very well be transferred aluminum, in which case Barkeeper's Friend could very well be your solution. Be very certain to test all of the surfaces first though, as this cleanser is acidic and can alter the finish of the tile if you aren't careful.
Best of luck,
Capua Custom Tile & Stone; Seattle, Washington
Member- National Tile Contractors Association
Card-carrying Schluter Systems installer
Certified WEDI Board installer
NuHeat radiant heating system "Qualified Installer"
"Working is more fun with cookies" ~ Tim Gunn
Last edited by Shaughnn; 06-01-2011 at 11:04 PM.