Pretty sure you can't get it on Lorveths path. You only gain access to the north eastern branches of the caves, saskia causes a cave in and blocks the rest of the tunnels off. Both entrances are locked until the siege even with Mottle's key, so getting there beforehand is not possible.
I agree 100%, and for this reason, I have removed the statement to the contrary; however if anyone can succeed in getting it on Iorveth's path, please don't hesitate to update the main page with the correct method.
I got it, you have to get it before that mission.
Also, it has been said that you need the "Catacombs Key", found on a dead dwarf in the TotF, in order to get the sword. I found this to be incorrect. In fact, the catacombs key is used to open a door that allows access to a tunnel that is directly across the chasm that you encounter while in the western end of the southernmost tunnel in which the sword is found. IIRC all that is there is some Nekkers, sone Rotfiends, and a chest with some trousers (Temerian Leather Galligaskins, +1 Armor, -20% Bleeding) and a few orens...
Apparently you can acquire the blood sword on Iorveth’s path, however both doors to the tunnel of the founders are locked, is there any way I can gain access to the tunnel of the founders before the Vergen besieged quest?
From what I can tell, it is only available on Iorveth's path on the Xbox. The only people I've seen that say they got it on Iorveth's path were either on the 360 or don't say what they were using. So far, nobody has given an actual description of how to get it.