How to dump a file stored in a sqlite database as a blob?

sqlite3 cannot output binary data directly, so you have to convert the data to a hexdump, use cut to extract the hex digits from the blob literal, and use xxd (part of the vim package) to convert the hexdump back into binary:

sqlite3 my.db "SELECT quote(MyBlob) FROM MyTable WHERE id = 1;"  \
| cut -d\' -f2                                                   \
| xxd -r -p                                                      \
> object0.gz

With SQLite 3.8.6 or later, the command-line shell includes the fileio extension, which implements the writefile function:

sqlite3 my.db "SELECT writefile('object0.gz', MyBlob) FROM MyTable WHERE id = 1"