[wrholt]

Anoche estaba borracho = "I was drunk last night". In most contexts in which someone might say this sentence, the fact that there was some moment when the speaker started being drunk and that there was (probably) some moment when the speaker was completely sober again is irrelevant.

Anoche estuve borracho = "I was drunk last night" sounds strange; the preterite explicitly invokes the moment of starting and/or ending being drunk, but the meaning of anoche is too vague to provide enough time reference to mark the moments of starting and/or ending, and the meaning of estar borracho also does not imply precise starting and/or ending times. So your listener may feel confused because they can't figure out why you are using the preterite, unless you intend to mean something like you were sober one moment and drunk the next moment, or you were drunk at some moment, and then completely sober the next moment.

The verb emborracharse = "to get drunk" is a different case, as the meaning explicitly describes the transition from being sober to being drunk as a one-time event. As a result, saying anoche me emborraché = "I got drunk last night" sounds natural.

On the other hand, the process of getting drunk is also not instantaneous, so it is also possible to say anoche me emborrachaba = "I was getting drunk", especially when you follow up with a description of some more-instantaneous event that occurred, such as cuando la vi entrar = "when I saw her come in".