From blithe +‎ -en.

blithen (third-person singular simple present blithens, present participle blithening, simple past and past participle blithened)

1. (transitive) To make glad or blithe; gladden.
   • 1895, John Galt, The Entail Or the Lairds of Grippy, page 304:

     When we entered the parlour, which had been so often blithened with the jocose spirit of its defunct mistress, we confess that our emotions were almost too great for our fortitude, […]

   • 1904, William Wells Newell, Words for Music, page 71:

     Let every life be content with its share
     Of dew, and sunshine, and blithening air.

2. (intransitive) To become glad or blithe; gladden.
   • 1924, Joshua Freeman Crowell, Florence Hathaway Crowell, Cape Cod in Poetry, page 16:

     The villages blithened when he came; […]

From Proto-West Germanic *blīþijan. Equivalent to blīthi (“happy”) +‎ -en.

blīthen

1. to be/become happy

• Middle Dutch: bliden
  • Dutch: blijden

• “blithen”, in Oudnederlands Woordenboek, 2012