Black grout staining

[cowgomoo]

Hi Y'all, just come across a problem of black grout staining a polished beige tile. The weird thing is is that it is not at all apparent at the tile edge but rather on the tile not directly touched by the grout during grouting but has received a grout covering during the washing off. It appears like a shadow and nothing seems to shift it, not elbow grease or grimex or anything. Has anyone come across this before and if so what solution did you find if any ?? Cheers you lkot, Cow.

[jay]

any pics might help