From The Jargon File (version 4.4.7, 29 Dec 2003):
BLT
/B·L·T/, /bl@t/, /belt/, n.,vt.
Synonym for blit. This is the original form of blit and the ancestor of
bitblt. It referred to any large bit-field copy or move operation (one
resource-intensive memory-shuffling operation done on pre-paged versions of
ITS, WAITS, and TOPS-10 was sardonically referred to as “The Big BLT”). The
jargon usage has outlasted the PDP-10 BLock Transfer instruction from
which BLT derives; nowadays, the assembler mnemonic BLT almost always
means “Branch if Less Than zero”.