'BLU' is a military abbreviation used by the USAF
. It stands for 'Bomb
, Live Unit', and is used to distinguish a bomb from a 'BDU', which is a 'Bomb, Dummy
The term covers a wide range of disparate exposive devices, and is generally forsaken for more specific terms (it dates from WW2, when bombs were much the same; modern-day munitions are more diverse, with CBUs and GBUs added to the mix, whilst conventional bombs tend to be called MK with a number). Nowadays, it unofficially stands for 'Bomb, large, unconventional', as the only things classified as BLUs tend to be oddities that don't fit elsewhere.
Such as the BLU-82 'Daisy Cutter', a 15,000lb blockbuster used in Vietnam and in late-2001 against the Afghan Taliban, the BLU-109, a hardened device for penetrating bunkers, the BLU-114, a classified, special-purpose bomb designed to disrupt electrical communications, and BLU-118 'thermobaric' bomb which kills people in caves.
Most infamous of the lot is the BLU-73 Fuel/Air explosive and its successors, terrifying devices designed to replicate the effects of a nuclear explosion - minus the radiation, and on a smaller scale - for a fraction of the cost.