ISO C99 says, of the isblank function (to which [:blank:] is related:
The isblank function tests for any character that is a standard blank character or is one of a locale-specific set of characters for which isspace is true and that is used to separate words within a line of text. The standard blank characters are the following: space (’ ’), and horizontal tab (’\t’). In the "C" locale, isblank returns true only for the standard blank characters.
[:blank:] is only the same thing as [\t ] (tab space) if you run your scripts and Awk and everything in the "C" locale.
Interesting, the GNU Grep manual describes both character classes as behaving as if you are in the C locale. I shouldn't have assumed it was the same as in the C standard!
