I recently had a situation where my fridge leaked water over a period of 2 years. I found it was a cracked pan and this is how the house was sold. In any case I didn't know it was affecting the base of the cabinet or toe kick adjacent cabinet and front facing toe kick. After much headache the hardie backer was removed. There was water damage on sub floor but a good sanding took care of this. The plywood toe kick a long with the oak veneer toe kick was removed and replaced. The tile is now set in. In preparing for the delivery of my new fridge I noticed a black discoloration on the toe kick located on the inside area of the oven space. The other side of this wall is a shower with fiberglass wall. There are no signs of water damage on the wall or inside cabinet. No damage that the floor guy can remember. Is the black discoloration on the particle board normal. There is no kitchen water source close by and the fridge was far enough.