General computing term.
Wikipedia: "Blittable types are data types in the Microsoft .NET framework that have an identical presentation in memory for both managed and unmanaged code… A memory copy operation is sometimes referred to as a 'block transfer'. This term is sometimes abbreviated as BLT... and pronounced 'blit'. The term 'blittable' expresses whether it is legal to copy an object using a block transfer."
See also: managed code and unmanged code.