Very blunt - referring to the fact that all nuns must vow to celibacy, and thus are not allowed to have sex. However, a lack of sex does not necessarily constitute a "bluntness" of the vagina (in fact, a blunt vagina would definitely be preferable to a sharp one, for ease of use), so the origin of the phrase likely originates from somebody looking for a funny sounding dirty rhyme.
1: "Fuck, this knife is blunt as a nun's cunt."
2: "...What? Oh, well, why aren't you using the other one then?"