Hi,
Ours is an old house. The kitchen floor is solid and tiled with flexible tiles - this was done by the last owner 20-30 years ago. About 10 years ago a black stain started to appear in one area where we suspect the fridge had defrosted and spilled water onto the floor. Suspecting that this was mould we scrubbed it with bleach and removed the stain. The stain came back and we decided to cover the floor with vinyl floor covering. Not long after, the black stain started coming through the vinyl!
Any suggestions what this might be and how to fix it?

If you have pipes that go through the floor a washer could have become loose and started to leak.

I have just been on a job where this happend, the owner had just covered it for 10years and the after effects were a nightmare.

You should be covered on your house insurance. But call a plumber (or a mate round to check it out).