hard to say exactly what the black marks are, but given the moisture could be mould or mildew. An alkaline cleaner may work. Failing that try some very dilute household bleach, leave it on for an hour, then rinse off.

The white marks may be a minor localized case of efflorescence. For such a small area I would try a proprietary acidic cleaner; one based on sulfamic or Phosphoric acid or one of it’s alternatives but DEFINAITELY NOT MURIATIC/HYDROCHLORIC. Try a little, diluted well with water. First pre wet the floor, then apply the acidic cleaner with water, leave a few minutes then lightly scrub. Pick up the liquid with a mop or absorbent towels. Then rinse with some more clean water then dry the floor immediately (remove all moisture as fast as you can). If it re occurs, repeat the process.

Hope this helps

Ian

I cannot remember if I answered you directly or not, apologies if I have not.

OK, to darken the tiles without using wax and without a caoting sealer, you would need an enhancing impregnator or oil (like boiled linseed oil).

However clealy you have some sealer on the tiles already, this would need to be stripped off with a good proprietray sealer tripper. The problem with then trying to use an enhacning sealer is that unless you can gurantee removing all the previous sealer, you risk not get the enhancer in evenly, leading to a patchy finish.

I would try a little boiled linssedd oil to see fi that works, rub it in and do not allow it to dry on the surface.

As for the satin finish – I think you will kind of achieve somthing like that by virtue of the feint traces of oil that inevitably get left on the surface, but it will not be like a matt finish coating sealer

hope this helps

Ian