Hey @hiccup! Let me start this post by saying that I am not, by any means, an expert in music or genres. I've been listening to music casually since I was 14, but I started to learn about it on a deeper level only last year.
I need to begin by explaining that Brazil is a large and diverse country. From the indigenous to the Portuguese Empire, from slavary to Italian and German imigrants, from dictature to freedom, all of it reflects directly on our culture, and so does our music. So yes, labeling musical genres can be tricky.
So 'MPB' literally stands for 'Música Popular Brasileira' (Brazilian Popular Music) and is derivated from Bossa Nova, which is derivated from Samba. That's why it can be confusing, especially for someone who is not from Brazil, to identify these genres easily.
MPB was created with the purpose of 'brazilianizing' Bossa Nova by blending traditional with contemporary elements. The ability to blend sounds from different regions of Brazil makes MPB hard to put on a box. One of the characteristics of MPB was the subliminal lyrics against the dictature (1964), which addressed social injustices, making MPB not only a musical genre but a movement as well.(PS: Not all artists used their music against the system, since some of them still defend the military dictatorship to this day.)
(...) does it describe a certain era of Brazilian music, and is it not used for modern music anymore?
The answer is yes... and also no.
Yes, MPB is a term that is used to describe Brazilian pop music during a certain period, especially during the dictatorship, when the genre was primarily derivated from bossa nova with the aim of 'brazilianizing' the genre by mixing it with traditional and folkloric elements from different regions. and often using its lyrics against the system.
And No. See, just like any other genre, MPB has suffered its alterations, especially after the end of the dictatorship (1985), by blending with foreign genres such as funk and even reggae.
So, is MPB a dead genre? No. It's different than it was before? Yes.
(...) what is your opinion on properly labeling Brazilian music with genres?
Labeling Brazilian music by genre is crucial.
As I said earlier, Brazil is a multicultural country, and our culture and music come from different places and people. 'Funk carioca', 'axé', and 'sertenejo' are worlds apart, and understanding them is really important.
The artists that I named in the aforementioned post will give you a clue about the Brazilian music I have in my library. (...) I just started listening to your suggestion Bem vindo, amanhecer by Saudade. It also seems good example of the 'genre problem'.
Now it gets tricky; for me at least, it's hard to explain why a genre is that genre. As a Brazilian who grew up listening to Brazilian music and its varied genres, it's easy for me to listen to 'Axé' and say, 'Oh, it's Axé, or listen to 'Sertanejo' and say, 'Oh, it's Sertanejo'.
This is something natural and automatic for me, so my best advice to you is to look into Brazilian sources of information, even if it's Wikipedia. Make sure that you are reading the Brazilian version.
Take a look at the albums that you have and not only the artist page, since a lot of them played a lot of different genres throughout their careers. In time, you will see the similarities and understand the genres.
(Which I would say is not easy since there is not a lot of information on the internet about the releases from that period of time.)
But of course, if you make me a list of the albums and artists that you have, I'll be more than pleased to help you with this since I am myself a Brazilian source of information.