So, Fire Emblem Three Houses turned out exactly as I feared...

[PAOerfulone]

One thing I should mention about the two Black Eagle routes is that one of them is hidden and this is how you unlock it... (Not sure if this is really a spoiler, but better safe than sorry.)

Spoiler!

The option to side with Edelgard and the Empire is the hidden route you have to unlock, otherwise you'll side with Rhea and the Church by default. First, you have to reach a C+ Support Rank with Edelgard by February 8. Next, you have to explore the Monastery some time between February 8 and February 22. There should be a opportunity to talk with Edelgard within this time frame where you learn about her tragic backstory. Then, you agree to go with her when she asks you to accompany her back to the Adrestian Empire to become the new Emperor. Once you do all of that, then you'll get the option after the battle inside the Holy Tomb in Chapter 11 to either Kill Her (Default Route) or Protect Her (Hidden Route). Choose the latter option and BOOM you get the Edelgard Route.