I don't know Bulgarian, but I happen to have a grammar here (by someone called Rå Hauge… doesn't sound very Slavic, but seems quite competent, although he may not use the same terminology as Kanes above). According to him, the aorist and imperfect are "in constant interplay in narratives about past events", with the aorist typically correlated with perfective verbs and the imperfect with imperfective (but the opposite combinations are also possible). "Bulgarian school grammar teaches that aorists answer to the question Какво стана? What happened?,while imperfects answer to the question Какво беше? What [state] was [there]?"
But then there is also the perfect, and when you add in the notion of status, things become very complicated! The simple aorist and imperfect have non-reported, "vouched-for" status, while the compound perfect has "neutral" status. However, there are also versions of the imperfect and aorist for "reported" status are compound forms, and in the case of the reported aorist, almost identical to the perfect (except that there is no auxiliary in the 3rd person).
Here is an example, three ways of saying "It rained last night":
През нощта валя [aorist, vouched-for] I was awake and saw it raining.But in actual reported speech, you don't normally use the reported forms: Той каза, че през нощта е валяло (He said that it rained last night).
През нощта е валяло [perfect, neutral] I see that the grass is wet this morning.
През нощта валяло [aorist, reported] Someone told me.
The use of these forms in different types of narratives, according to Rå Hauge:
folk tales, jokes, anecdotes: reported forms
modern fiction: reported forms almost never used in 3rd person
non-fiction and news reports: too complicated