[kazinator]

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.

[nerdponx]

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!