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!
Oh, it's like in German! The passive form in German is formed with the verb "Werden" which, among other things, means "To become".
I just had that rejected. I think it should be okay so I will report it.
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.