BBL, BibTeX Generated Bibliography File (.bbl)
The BBL Misc file format
BibTeX, which is a program that allows users to create bibliographies for writing academic and scientific work, creates plain text files that are saved with the BBL extension. These files contain references that are defined in a specific typesetting language which is used by LaTeX programs. They are used to easily cite references and sources when writing papers, reports, essays, and the like using LaTeX and the LaTeX document format (i.e. TEX).
Technical details of BBL files
BBL files can be created manually by a user of BibTeX. However, it's more common that the program creates the file itself on the basis of a BIB file that stores a bibliography database. The BBL file then references the entries in the corresponding BIB file.