stains on sandstone pavers

We recently laid some sawn sandstone stepping stones using cement and have surrounded half of the path so far with blue metal gravel. We temporarily placed some blackbutt planks of wood on the stone, it rained and the next day there were some blue-grey coloured stains on the stone. Most likely explanation is that tannins from the wood leached onto the stone, but the why they formed a blue colour (rather than brown tannin) is really odd, and suggests some unusual chemical reaction between the stone and the tannins, or also involving any cement leachates rising up through the stone. It occurred on sandstone pavers both surrounded and those not yet surrounded by blue metal, so it doesnt seem likely to be blue metal caused.
Any thoughts on why this colour, and how best to clean the sandstone?
many thanks!