A oscuras, a tientas, etcétera
 

Hi,

I've got a growing collection of such use cases:

a oscuras
a tientas
a ciegas
a cuestas
a medias

I've got two questions:

1. What's the name of this grammar thing?

2. Are they all set phrases of an established list, or can one make them freely?

They are instances of locuciones adverbiales (adverbial phrases). You could not make them freely.

There are many starting with the preposition a. There are many more constructed in a different way:

de cuando en cuando = every now and then
de cabo a rabo = from head to toe
en mi vida = never

Many of them may be replaced by a proper adverb:

a horcajadas (loc. adv.) = acabalgadamente (adv.)= astride (prep.)
sin duda = indudablemente = undoubtedly

So the family is wider than I expected :)

Another, more specific question:

a oscuras

What is a and what is -as?

a oscuras/obscuras = in the dark

I don't know the reason but many of these expressions using adjectives become nouns are plural and feminine:

a ciegas = blindly (literally)
a oscuras

but other expression use nouns in plural (or singular) of any gender

a tientas = blindly (it literally means "by using the sense of touch")
a gatas = crawling (on your hands and knees)
a pedazos = in pieces
a gusto = comfortably

Thanks! The meaning is obvious, usually. Except for a gatas :) . Do they mean cats?

Yes, it comes from gato. It's used to suggest "in a quadruped way".

There is also a very expressive verb gatear :p