"The doors were opened yesterday."
Translation:Dørene blev åbnet i går.
I think you are confusing the past perfect with the past passive being used in this sentence which is always formed using blev (or by adding an s to the end of the regular past). In English this could also be translated as The doors were beeing opened yesterday to better express it is a passive tense.
Dørene var åbnet i går. (where åbnet is an adjective?) does seem to be a valid Danish sentence meaning "having at some earlier time been opened and thus being in an open state at the time spoken about" But there is also Dørene var åbne i går. where åbne is an adjective and it is a simple description of the state without any further information. I am still a bit confused. The sentence is definitely also passive or else it would be Manden har/havde åbnet dørene i går. (edited to remove a silly mistake...)
I have come to the following conclusion:
- Dørene blev åbnet i går: the doors were opened (by somebody) yesterday;
- Dørene var åbne (and not "åbnet/åben/etc) i går: yesterday the door were open (they are not anymore).
In the first sentence åbnet is a past tense of the verb at åbne, in the second sentence it is an adjective (åben/åbne). Do you both agree?
Edit: but I believe in all those cases you can find on ordnet.dk/korpusdk "var åbnet" is used as a synonym of "blev åbnet", in support of my first comment: blev and var can both be used as auxiliary verbs with at åbne.