"Blood diamonds" are mined not only in Sierra Leone, but also in war-torn countries like Botswana and Angola. I'm not sure if Sierra Leone diamonds are structurally different or not, but I do know that A LOT of these "blood diamonds" are smuggled across international borders into the markets of other countries, so there's often no way of determining their points of origin. Presently, I believe, there are no domestic certification procedures and norms to regulate these things (correct me if I'm wrong, by the way.) So, it's hard to bypass this whole issue with "well, I just won't buy a blood diamond, then...", 'cause you never know what you're getting.
On the up side, I know De Beers, various NGOs, and even the UN have started to crack down on the smuggling (and mining!) of blood diamonds, so hopefully things will get better.