Is it correct to use the comparative adjective "blacker?"

Solution 1:

The reason that some dictionaries do not contain "blacker" and "blackest" is that "black" is considered to be an absolute adjective. This is the same reason you are not supposed to say "more perfect", "more unique", "whiter", "deader", or "fuller". See, for example, this web page.

There are lots of people who do not abide by this "rule". For example, Shakespeare, Procol Harum and the writers of the United States Constitution.

Solution 2:

What dictionary are you using? With a quick online search, I see the word "blacker" in these:

• http://dictionary.reference.com/browse/blacker
• http://en.wiktionary.org/wiki/blacker
• http://www.merriam-webster.com/dictionary/black (only the racial definition)
• http://oxforddictionaries.com/definition/english/black lists "blackest" in one of its senses, so "blacker" must exist.
• http://www.memidex.com/blacker