From Just Solve the File Format Problem
From Wikipedia, an Opaque Binary Blob is a term used in network engineering and computer science to refer to a sizeable piece of data. 
According to Wikipedia also, Android Operating Systems starting at 2.3 (Gingerbread) started to use OBB files in different capacities.
One capacity an OBB is used in is in the XAPK distribution format for Android applications. OBB files can be unpacked as it is a ZIP archive like other packages used for Android App distribution.
The Android developer JOBB tool can be used to package files as OBB and optionally encrypt them.