A BLEND1 file is a backup file automatically generated by Blender, a free 3D modeling program. It stores a copy of the previously saved version of the currently opened .BLEND file and is used for recovering the previous version of a BLEND file. BLEND1 files can be renamed with the ".blend" extension and opened normally.
BLEND1 files may be accompanied by other ".blend2," ".blend3," etc. files. These files are generated based on the number of allowed save backups the user specifies in the application preferences. Note that ".blend2" is an older version of the Blender file than the ".blend1" file.