Verb form of "to blacken" versus "to brown" [duplicate]

The old -en Causative/Inchoative suffix is no longer productive in Modern English, but it's still part of quite a few verbs, including redden, blacken, whiten, darken, lighten, weaken, broaden, gladden, madden, and sadden, among others.

But it doesn't attach to other colors or adjectives, as you note for brown; consider also the awesomely wrong verbs *purplen, *orangen, *greenen, and *bluen.

Interestingly, there is also an en- prefix with the same meaning -- cause to be or come to be -- as in encourage, enlarge, enable, ennoble, embody, emblazon, etc. These can occasionally be found together, as in enlighten, embolden, enliven, and enhearten.