Bless doesn't have a setting in the Interface menu to enable Invert Mouse in either the Korean or Japanese versions of the game. In the streams that covered the recent press event for the NA release, it doesn't look like that option will be included when Bless enters Early Access on the May 28th. Since this option is critical for my enjoyment, I decided to do some research and see if there was a way to enable it via customization in the ini files since the engine is based on Unreal 3. After a few hours of Google investigating and some good ol trial & error, I'm happy to say there is a way to enable this option by adjusting the BLinput.ini file.
Here's how to do it:
1.) Open your Bless root directory to the "Config" folder. It should be under GameOn -> Bless -> BLGame -> Config
2.) Select the "BLInput.ini" and open it using Notepad.
3.) Insert the line bInvertMouse=true
4.) Save the file and launch Bless
This was tested using the Japanese client but I would assume this will also work in the NA release as well.
Enjoy!