I am not an expert by ANY reach of the imagination, but I believe "blev" is used here because it is discussing the result of something, or "what became of it". I found an article discussing this, using this example:
"Min cykel blev stulen när jag var på semster.
(My bike got stolen when I was on vacation./My bike was stolen when I was on vacation.)
In Swedish we have to use “blev” in this case since it expresses the transition from the bike not being stolen to it being stolen. If I said “min cykel var stulen när jag var på semester” it means that my bike was stolen only when I was gone but the thief got second thoughts and returned it."
Here is the article for further reading. http://www.thelocal.se/blogs/theswedishteacher/tag/bli/
I hope this helps!
The passive form in swedish, at least from what I know, is formed by adding -s as an ending. In this sentence it doesnt resemble the passive form but the "blev" just means "became", it just describes that the state of something has changed.
Like in the sentence "Min cykel blev stulen" it means "My bike became stolen" (I know this sentence isnt correct english, but I just want to make it clear). So basically the sentence just expresse that the bike changed its state to being stolen.
I start to wonder two things about the verb att bli. 1. I think I have only seen it as copula until now, but can it also have other gramatical uses, like a passive auxillary? 2. Can it have more meanings than just to be or to become? For me it feels like it is being thrown around a bit.