I am 100% new to Blender and to the Blender Manual. Could anyone tell me what a blend file is please. It is discussed at length in the manual except for explaining what a blend file is.
The .blend files are the project files of Blender.
They are binary files with the .blend extension as in fileName.blend. Blender saves its data as .blend files and can only open .blend files.
Other 3D files types are not "opened", but imported.