The blur effect is just a slightly modified version of the vignette shader found in the Godot demo repository. If I remember correctly, the shader is stored on a TextureRect in the Level_<number here> scenes.
Soon I will be releasing a update to fix a few bugs, and I will be remapping the controls (but not adding configurable controls, which will come later). Sorry to hear that it froze your computer. Do you mind telling which OS you used, and (if you know) roughly where it froze? I have a hunch that the freezing is occuring either at level 4.1 (the ‘secret’ level) or level 5 (where 3D is introduced), but that is just guessing on my part.