I've had a client site recently hacked and displaying a "blackhole exploit kit" being the issue, I'm by no means a security expert as I've seldom had to deal with such issues but I learning quickly, any advice on this particular exploit? any help would be great.

Just to chime in as a general rule my WP sites have been secure, out of a very large number of site I've worked on and monitor this has only happened twice, on the site I've mentioned and previously with the timthumb hack (again - not really WP's fault) from both personal experience and the stats I could find, WP is fairly secure as far as al of its competitors which I think is largely due to its active and well, massive community.