The blend file format is not a true file interchange format, rather it dumps internal data structures from directly from memory to disk. A blend file’s structure may therefore be unique to each version of Blender. Despite this, blend files are both backward and forward compatible between versions, and between different hardware and operating systems. This is made possible by the addition of metadata, known as Struct DNA, that allows conversion when loading the file.
Since no standard blend file format specification exists, the Blender source code must serve as the definitive specification. Blender also allows option while saving to "compress" file with gzip, retaining .blend extension.