Why are 'blueish' and 'bluish' both considered correct spellings?

My nine year old son fought hard on this and is taking a stand on spelling bluish as blueish. I'm certain his teacher will mark it as a spelling error in his writing... Several dictionaries have indicated both spellings are acceptable. A character in his story says

What is that blueish white piece of ice floating in the freezing cold Antarctic waters?

Both the NOAD and the OED define blueish simply as "spelling variant of bluish." (See also bluish on the Oxford Living Dictionaries.)

Looking for both the words in the Corpus of Contemporary American English, there are 7 sentences containing blueish and 559 sentences containing bluish; for the British National Corpus, the numbers of the sentences are respectively 4 and 55.

I don't think it's correct to say that both are correct spellings. After looking through N-gram data, and querying the venerable Fowler & Crystal's A dictionary of modern English usage, I find that bluish is clearly the preferred form.

Here's what Fowler & Crystal have to say:

The only satisfactory rule, exceptions to which are very few, is this: If the suffix begins with a consonant, the mute e is retained; if the suffix begins with a vowel, the mute e is dropped. Applying this [ ], we get (with the wrong results in italics, as a basis for exceptions): stalish; loving; milage; livable; stirving; excitable; timous; movable; likely; dotard; judgement; hinging; singing; gaugable; laughable, noticeable; mousy; changing; hiing; gluy; duely; bluish; wholely.

Another grammar book, A Survey of English spelling by a Edward Carney, explains the reasoning behind the rule:

The {-e} has to be kept when it is a marker of the pronunciation of the previous consonant as in gaugeable, manageable, noticeable, traceable. A spelling *{noticable} would invite {c} = /k/ before the as in practicable. So, singeing with /ndʒ/ is kept different from singing with /ŋ/.

Thus, the implication is that as there would be no ambiguity between the pronunciations of blueish and bluish, the preferred spelling is the latter.

Here are the Google n-grams for usage; first for American English:

Next, British English:

It seemed that blueish did seem to enjoy some popularity as the preferred form for some years in both America and Britain; peculiarly, between the years of 1786 - 1792 in Britain, and in 1788 - 1792 in America, Google Books shows that blueish enjoyed the advantage, being particularly applied to describe the color of dyes and minerals. (Here are some citations from 1788 - 1791.) The nascent field of chemistry seems to have preferred the odd spelling for some reason or the other. But as you can see, despite that brief respite, since then bluish has reassumed its dominant position.

I'm not convinced there's a strong case for saying blueish is outdated or to be avoided for some other reason. It's just the less common of two variants, but personally I don't really have a preference.

I think an even more finally-balanced example is clueing / cluing, which occurs frequently in crossword-related writing. In that particular case I slightly favour including the e, though I certainly wouldn't take issue with it either way.