Was the block starched? I have found that often I have scorched the starch, not the fabric. In this case the scorch will wash out later, when the quilt is washed. I have also had it happen that starch got on the iron and scorched, and then was transferred to the fabric. Again, in this case it will wash out with the starch.
If the fabric is scorched -- meaning, the fabric is actually burned brown -- I'm not sure that anything will bring back the original color.