Original specification:
http://wc3lib.org/spec/BLP.txt

Additional information for BLP JPEG data:
https://en.wikipedia.org/wiki/JPEG#Syntax_and_structure
gives information about JPEG markers

see
/usr/share/doc/libjpeg-turbo-1.3.0-r3/libjpeg.txt.bz2
section "Abbreviated datastreams and multiple images"

An original file of Blizzard using JPEG compression and BLP1 (TeamColor00.blp) says:
FIF..........ÿþ.*Intel(R) JPEG Library, version 1,5,4,36.ÿÛ.C.........

markers found are:
FFD8	// Start Of Image
FFE0	// Application-specific
FFFE	// Comment
FFDB	// Define Quantization Table(s)
FFDB	// Define Quantization Table(s)
FFC4	// Define Huffman Table(s)

FFC0	// Start Of Frame (Baseline DCT)

MIP maps (4 mip maps):

FFDA
FFD9
FFDA
FFD9
FFDA
FFD9
FFDA
FFD9