How many letters does the top answer for “to grow black or dark.” have?

The most common answer is blacken (7 letters).

Crossword clue: to grow black or dark.

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

blacken 7 letters web-def

make or become black

a very dark black 50% similar → ebony

dark as a pitch; pitch-black. 50% similar → pitchdark

of a very dark black 50% similar → ebon