Point blank range is a range so close to the target that one cannot miss. The original sense, however, is slightly different. It refers to the range close enough that one can aim an artillery piece directly at its target without adjusting for the fall of the shot. (Gravity causes the shot to start falling in a parabolic arc the moment it exits the barrel of a gun, but at very close, i.e., point blank, ranges the fall is negligible.) The English term probably comes from the Middle French de pointe en blanc, which appears in 1569. English usage appears only a couple of years after this. From Leonard Digges’ 1571 A Geometrical Practise Named Pantometria:
Hauing a table of Randons made, mounting your peeces accordingly, no vessel can passe by your platfourme (though it be without poynte blancke) but you may with your ordinaunce at the first bouge hir and neuer bestow vayne shotte.
The words point, meaning small bit or degree, and blank, meaning void or nothing, were probably chosen because the amount of adjustment needed to hit the target at such ranges is negligible. The phrase is often construed to refer to point, to aim, and blank, the white bullseye. This does not comport either with what we know of the early uses of the phrase, nor with how verb-noun phrases are formed in English. In almost all cases, the nouns are the direct objects of the verb, as in cut-throat or stop-gap. This is not the case with point blank.
(Source: Oxford English Dictionary, 3rd Edition)
Copyright 1997-2013, by David Wilton