sorry bro not sure about your specific situation, i read a lot of stuff before doing it, as far as i can tell blackra1n wouldn't work for you, for me however having bought a brand new 3gs already loaded with 3.1 blackra1n worked fine i just updated to 3.1.2 and ran it done in a minute flat!
check this link out may help you a little as theres lots of people who've left feedback on blackra1n... http://www.redmondpie.com/jailbreak-iphone-3.1.2-firmware-with-blackra1n-zni327/