Spam primarily sells shitty stuff for unsophisticated people: if you're bright enough to obfuscate your address, even with [AT] ... [DOT], chances to sell you herbal Viagra or to phish your SSN are very low, there's little point collecting your e-mail address.
OTOH, when spammig your blog comments, the primary audiences are readers and bots, so your net-literacy isn't a disincentive to spam.
YMMV
Or are you simply using a simple math question (un-captcha'd)?
I've been experimenting with my own db of questions that can be answered with a word - including simple arithmetic and simple reasoning such as "Which is warmer? Hot or cold?" - and finding almost no spammer cares enough about my sites to develop a decoding table for comment submission [that would of course change if I were Amazon, etc.]
All of this is moot, of course, if some spammer were to care enough about auto-registering/emailing my site to get porn-visitors to solve the questions/captchas in order to "see more".
The formmail script then checks for the right answer.