“Blithe” is an adjective and it means unconcerned (despreocupado). It can also mean “carefree” (risueno) or “light-hearted” or a very casual attitude. Some examples:
Do not be so blithe (no seas tan alegre).
Mr. Sanchez shows blitheness towards the economy. Here “blitheness is the noun.
The adverb is blithely (alegremente). An example:
Mr. Sanchez seems blithely unconcerned about the Spanish economy (…tiene un aire despreocupado sobre la economía española).
So, sometimes we are blithe about something, meaning that we behave blithely, showing blitheness.