To answer the question: "Where does the line between what is acceptable to call plagiarism and musical "style" come in to play?" I have to say that unfortunately, pragmatically, it comes down to what you as a plaintiff can prove in court. It really does come down to the law.
Disclaimer: I am not a lawyer, and I am not giving any legal advice, which in any case would meet different requirements under the laws of different nations.
The over-simplified answer is that a melody is copyrightable, and a lyric is copyrightable, whereas a chord progression is not copyrightable and the finer points of an arrangement are not copyrightable either.
Anyone can register a copyright on a song or piece of music. If that person believes that someone else has infringed on their copyright, it is up to that copyright holder to sue the other party and to try to prove in court that copyright infringement has occurred. To do so successfully, the copyright holder would have to prove several things, among them that the plaintiff's piece holds a valid copyright that has not expired, that the piece copyrighted was unique, that the defendant had copied the characteristics of the plaintiff's copyrighted piece to a degree that constitutes infringement in the opinion of the judge or jury, and also to prove that the defendent must have heard the plaintiff's copyrighted song before the defendant wrote the song that it is alleged had infringed on the plaintiff's copyright.
If there are indeed hundreds of different copyrighted blues or country songs in the world with the same chord progression and song structure, then a plaintiff cannot successfully prove in court that someone else has infringed on his one copyrighted song with regard to the chord progression and song structure alone. The defendant would successfully argue that they were copying the characteristics of many songs, but not the specific song they were being sued for that day.
In other words, if you assert that you have a copyright, you have to defend it yourself in court with your own lawyers and your own money. The burden is on you to prove that what you have is entirely unique and a specific person has infringed upon the entirely unique characteristics of your property in a distinct way, and that neither you nor the other person got your ideas from anywhere else.
The outcome of this, over many years of court trials all over the world, and reflected in the writing of laws all over the world, including international copyright treaties, is that it is easy to prove that a melody or lyrics are distinct and defendable in court. However, generally, things like a chord progression, a song structure, and a rhythm, all by themselves and not in conjunction with a certain melody or lyrics, are not things that can be asserted in court to be copyrightable. Nobody could win a case with that.