New Tiled Floor Problem, Advice Please?

[Italy]

same

Lets not over complicate things here jeez.
Ask them to some and re grout the dark patches and see how it goes. It will probably fix your problem. You didn't mention generally patchy grout so I assume you don't want the whole lot re grouting.
It looks like some dirt or other on the grout that's all, there has probably been all sorts of trades walking all over it after its been tiled

You found this a lot easier than I did to explain! This is exactly what I meant! (I think)

same