from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Exhibiting great detail: a blow-by-blow description of the accident.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. Detailing every action or occurrence completely.
- n. An account, description, or commentary including every detail of the action or event.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- adj. providing great detail
Probably from the detailed description of a fight or boxing match, where a "blow" means a hit or a landed punch. (Wiktionary)