There's a confusion of terms here.
I don't think open source is the correct term as you'll still be required to own Blood to run Bloodxl.
This has nothing to do with the source. It is true that you need the original data from Blood to use BloodXL. But that has no bearing on the extendability of the engine or anything like that.
The source code will be released. Initially it will be a game-only source code release (i.e. source code for the game DLL's) but the main engine will follow once I'm happy with the state of the code (probably when the software rendering mode is released).
In the meantime you'll have access to the game code and all the scripts. Since enemies, UI, weapons and gameflow are handled through scripts the games can be heavily modified even without the source code.