What are some examples of spam content on your blogs? (Sometimes it's hard to tell whether something is a real comment or spam!) Something like this: "This is typical of this kind of action. I would prefer we looked at it from a softer angle to be sure we have it under control." I would have let this one through if it hadn't come up for EVERY SINGLE one of my posts!
Check whether the comment comes with link. If the comment comes with link, with keywords as name, then most probably are spam.
But most of the time you have to go on manual checking because akismet is filter genuine comment too.
I usually get ones that say somethin like "I find this article really useful, thanks for sharing" etc. They're always really vague and pointless. They could apply to any blog post because they're so vague and generalized. People are really lazy, I hate it.