Use the adjective unbleached to describe something that looks like it might be chemically lightened but actually isn't. If you're a baker, you might know that regular, all-purpose flour is often chemically treated — while unbleached flour is not.
The word unbleached comes from adding the prefix un- ("not") to bleached, which stems from a root word that means "shining white." You'll almost always find this adjective describing things that might typically be bleached. For example, fabric and paper are often lightened with chemicals, but unbleached cotton fabric and unbleached writing paper are not.