Fire stains

If fire stains were added, there would be more dark variants of different burnable blocks. You would get this variant by putting out a fire. If a block was on fire, and the fire was put out before the block was destroyed, then the block would be stained by the fire. It would make wooden planks ash grey, grass (which should be more burnable) would blackish grayish, and logs would be darker too. I think that these blocks would be cool and would give builders more dark blocks to choose from. Plus, to get these blocks, you would need to start a fire and put it out, which not a lot of people do on purpose. Also, burnt logs could be crafted into charcoal.