»im Dunklen tappen« vs »im Dunkeln tappen«: What is the difference?

Question

According to the inflection chart of the adjective dunkel, its neuter dative form for the weak inflection is dunklen. But both of the following phrases seem to be used. I wonder what is the difference between the two. Does it come down to a personal preference?

im Dunklen tappen

im Dunkeln tappen

Answer 1

We have:

1. im Dunkel: This is the noun Dunkel (elevated form of Dunkelheit).
2. im Dunklen: This is the nominalized form of the adjective dunkel.
3. im Dunkeln: This is probably a set expression, alternative to 2. It might as well be an alternative (archaic?) dative of Dunkel. The Duden lists this phrase only with the adjective, not with the noun.

I think:

• im Dunklen is mandatory when talking about beer or something else named das Dunkle.
• im Dunkeln is mandatory (at least: good style) when it means the unknown, obfuscated.
• Meaning the absence of light, both forms seem appropriate. Though im Dunkeln sounds much more idiomatic to me, im Dunklen is still fine if you want to emphasize, that you do not use it figuratively:

• Die Polizei tappt im Dunkeln.
• Er tappt im Dunklen nach der Taschenlampe.

Answer 2

Those are different words:

• das Dunkle
  This is a nominalized adjective, derived from the adjective dunkel. In English it is "the dark".

  Das Dunkle schmeckt mehr nach Malz als das Helle.
  The dark tastes more like malt than the light.

  Note, that in English nominalized adjective are very rare, so in the English translation of my example dark and light still are adjectives, because they both will be interpreted as attributes of the omitted noun "beer". But in German »das Dunkle« and »das Helle« will be understood as nouns, and this also is the reason why you have to write them capitalized. In English you can find nominalized adjectives in phrases like »the poor and the rich«.

  Note, that nominalized adjectives (das Große) and nominalized verbs (das Gehen) are nouns (with all rights and obligations of nouns). They are not adjectives/verbs!

• das Dunkel
  This is a "normal" noun, not a nominalized adjective. Etymological it also evolved from dunkel, but grammatically it is an independent word. In English it is "the darkness".

  Siegfried reitet in das Dunkel.
  Siegfried is riding into the darkness.

If you tap in the dark, it doesn't matter much if you mean "tap in the dark" or "tap in the darkness". It's quite the same.