By Blood Alone does for aviation what earlier DLC had already done for ships and tanks, and uses the occasion to overhaul Italy and add the peace conference system the community had been asking for for years.
What New Content It Adds
The pack splits its additions between a rebuilt tree for Italy and a couple of system-wide mechanics that change any game where you design aircraft or win a major war.
Nations: Italy, Ethiopia and Switzerland
Italy gets a completely overhauled focus tree with more room to maneuver on both the fascist path and its alternatives. 
Ethiopia and Switzerland get playable content of their own with real substance for the first time — before this, there wasn't much there.
Mechanics: Custom Planes and Peace Conferences
The plane designer lets you build fighters, bombers and naval planes piece by piece, just like ships already worked. But what actually changes the feel of playing is the peace conference: instead of an automatic resolution when you win the war, you now negotiate what you keep, what you liberate, and what you hand to your allies. It's rounded out by unit medals, which boost morale and performance for veteran divisions, and an economic embargo system.
Is It Worth It?
If you like customizing equipment down to the last detail, the plane designer holds up just as well as the naval or tank ones. And peace conferences, being a general system mechanic, change any game where you win a major war — you don't need to touch Italy to feel it.